This lemon blueberry cake sandwiches bright, homemade blueberry jam between layers of tender lemony cake, striking the perfect balance between sweet and tangy. A lemon soak and lemon frosting streaked with more blueberries reinforce this classic flavor combination.
Preheat the oven to 350°F with a rack in the center. Line the bottom of three 6" round pans or two 8" round pans at least 2" deep with parchment; grease the parchment and the sides of the pan.
To make the cake: In the bowl of a stand mixer fitted with the flat beater or working with an electric hand mixer in a large bowl, mix the sugar and lemon zest together until the sugar is moist and fragrant.
Add the flour, baking powder, and salt. Mix on low to combine.
Add the butter, mixing on medium-low speed until the mixture looks like damp sand, 1 to 2 minutes.
Add the whole eggs one at a time, followed by the egg whites, beating on medium speed. Scrape the sides of the bowl after each addition.
In a separate measuring cup or small bowl, combine the milk, vanilla, and lemon emulsion. Add this mixture to the mixing bowl in 3 additions, beating the batter at medium speed for 1 minute and scraping the bowl after each addition. The batter should look smooth, fluffy, and off-white in color. If not, beat for an additional minute at medium speed.
Portion the batter evenly into the prepared pans, gently smoothing the top. Each 6" pan will hold approximately 1 2/3 cups (about 337g) of batter. Each 8" pan will hold approximately 2 1/2 cups (about 506g) of batter.
Sprinkle the blueberries on top of the batter, distributing them evenly among the pans.
Bake the lemon blueberry cake for 30 to 35 minutes, or until the cakes just begin to pull away from the edges of the pan and feel firm to the touch.
Remove the cakes from the oven and place on a rack to cool for 10 minutes while making the soak.
To make the soak: In a small bowl, combine the water, lemon juice, and confectioners’ sugar; whisk to dissolve the sugar. Brush or spoon about 1 1/2 tablespoons (17g) of soak onto the top of each 6" cake layer or 2 tablespoons (about 25g) on each 8" layer.
Run an offset spatula or dull knife around the edge of each pan, and turn the cakes out onto a rack to cool completely. Brush 1 1/4 tablespoon (about 17g) of additional soak onto the bottom of each 6" cake layer or 2 tablespoons (about 25g) on the bottom of each 8" layer so both sides are soaked. While the layers cool, prepare the filling.
To make the blueberry filling: In a small saucepan, combine the blueberries, lemon zest and juice, and sugar.
Place the saucepan over medium-low heat and cook, stirring occasionally and mashing some of the blueberries with a fork, until a thick, chunky jam forms, about 5 minutes. Remove the pan from the heat and set aside to cool completely.
To make the frosting: In the bowl of a stand mixer fitted with the whisk, beat together the butter, vanilla, lemon emulsion, and salt until fluffy and pale yellow in color.
With the mixer on low speed, slowly add the confectioners’ sugar, lemon zest, and lemon juice until combined. Then gradually increase the mixer speed to high to combine and aerate the ingredients, about 2 minutes, or until the mixture is fluffy.
Fit a large pastry bag with a plain round tip, about 1/2" in diameter (we like Ateco #807). Add about 1 1/4 cup (about 260g) of frosting to the bag (no need to be precise here), leaving the rest in the bowl.
Measure and set aside 1 tablespoon (about 18g) of blueberry jam to use on the outside of the cake. The rest will be the filling.
To assemble the cake: When the cake layers are cool, level them by cutting the dome off each layer (this is called "torting the cake"), if necessary. Place a quarter-sized dollop of frosting in the center of the serving plate to keep the bottom layer from moving. Place the first layer of cake bottom-side up on the plate. Set the plate on top of a turntable or lazy Susan. Tuck strips of waxed or parchment underneath the edge of the cake to keep the plate clean during assembly.
Pipe a 1/2"-tall circle of frosting (a dam) around the perimeter of the cake. This will keep the blueberry jam in place.
Fill the circle of frosting with half of the remaining blueberry jam (about 1/4 cup or 85g) if making a 6" cake or all of it if making an 8" cake. Use an offset spatula to smooth it into an even layer, spreading it to the edges of the frosting dam.
If making a 6" cake, repeat this process with the second layer of cake.
For the final layer, center the remaining cake on top, bottom-side up, and press it gently into place.
Use any remaining frosting in the piping bag and/or frosting from the bowl to apply a thin layer of frosting to the top and sides of the cake (a crumb coat), then set it in the refrigerator to chill for 15 to 20 minutes. Cover and set aside the unused frosting at room temperature.
To frost the cake: Remove the cake from the refrigerator and cover the cake with a final, even layer of frosting.
For the most refined look, pass the tablespoon of reserved blueberry jam through a sieve to move out any large bits of blueberry. Then randomly dab the sieved jam onto the sides and top of the cake. (For a more rustic look, skip the sieving and simply dab the chunky jam onto the cake, expecting some large bits to fall off in the process.)
Use an offset spatula or a bench knife to smear the blueberry jam on the sides by holding it at a straight angle against the cake and gently spinning the turntable.
Repeat this smearing process on the top of the cake. Carefully remove the strips from under the cake. Slice and serve the cake at room temperature.
Storage instructions: Store leftover lemon blueberry cake in the refrigerator, covered, for up to 3 days. Freeze, well wrapped, for up to a month.
Baking gluten-free? Follow the lemon blueberry cake recipe as written using our Gluten-Free Measure for Measure Flour 1:1 in place of cake flour. After sprinkling the blueberries onto the gluten-free cake layers in step 8, stir/press them gently into the batter before baking. (The blueberries don’t sink into the cake batter as they do in the batter made with conventional wheat.)
Short on time? Make the blueberry jam filling up to 2 days in advance and store, covered, in the refrigerator. Or use Blake Hill Blueberry Pie Filling or your favorite store-bought thick, whole-fruit blueberry jam as a 1:1 volume replacement. The cake layers can also be baked, cooled, and refrigerated up to 2 days in advance. Wait to apply the soak until ready to assemble.