Neatly walking the line between breakfast treat and dessert, this crumb cake sports an intensely apple-flavored layer under its buttery streusel.
To make the streusel: Whisk together the flour, sugar, salt, and Apple Pie Spice. Add the vanilla to the melted butter and add all at once to the dry mixture. Mix until the crumbs are evenly moistened, breaking up any large clumps with your fingers or a fork. At first, the crumbs will feel very wet; this is OK. Set aside.
To make the filling: Place the apples, boiled cider, 6 tablespoons (74g) granulated sugar, and Apple Pie Spice in a medium saucepan. Cook over medium heat for 10 to 15 minutes, stirring occasionally, until the apples are tender.
In a small bowl, whisk together the remaining 2 tablespoons (24g) granulated sugar and Instant Clearjel. Once the apples are tender, add to the saucepan and stir until the sugar is dissolved and the mixture has thickened. Remove from the heat.
Preheat the oven to 350°F with a rack in the center. Grease a 9" x 13" pan. Line the pan with a parchment sling and lightly grease the parchment.
To make the cake: In a small bowl, whisk together the flour, baking powder, baking soda, and salt and set aside. In the bowl of a stand mixer fitted with the flat beater attachment or working in a large bowl with an electric hand mixer, beat the butter and sugar together until smooth.
Beat in the eggs, one at a time, and the vanilla. Add one-third of the flour mixture to the batter, beating gently to combine. Gently beat in half the sour cream. Mix in another third of the flour, then the remaining sour cream. Beat in the remaining flour. Scrape down the sides and bottom of the bowl, and continue mixing at low speed until the batter is smooth.
To assemble: Transfer the cake batter to the prepared pan and use a small offset spatula to spread evenly. Dollop the apple mixture evenly over the batter and use a small offset spatula to spread evenly. Aiming for 12 small piles of apples evenly spaced across the pan makes it easier to spread. Top with the streusel, breaking up any large clumps that have formed.
Bake the cake for 40 to 50 minutes, until a paring knife inserted into the cake base comes out clean. Remove from the oven and cool on a rack before dusting with confectioners’ sugar and serving.
Storage information: Store the cake, well wrapped, at cool room temperature for several days; freeze for longer storage.
