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Preheat the oven to 350°F with a rack in the center. Line a standard muffin pan with baking cups and spray the pan and cups with nonstick spray.
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To make the cake: Place the butter in a medium saucepan and set over medium heat. Once the butter melts, continue to cook the butter, stirring frequently and watching closely, until the butter foams, browns, and smells nutty and toasty, 3 to 5 minutes.
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While the butter melts, in a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt.
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Transfer the browned butter to a large bowl, add the brown sugar, and whisk until well-combined. Add the maple syrup, eggs, vanilla, and maple flavor and whisk until lightened in color and creamy-looking.
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Add 1/3 of the dry ingredients to the batter, beating gently to combine. Gently beat in half the buttermilk. Mix in about half of the remaining flour, then the remaining buttermilk. Stir in the last of the flour.
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Portion the batter evenly into 12 wells of the muffin pan (each well will be filled most of the way); a muffin scoop works well here.
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Bake the cupcakes for 16 to 20 minutes, rotating the pan once halfway through, until a toothpick or paring knife inserted into the center of a cupcake comes out clean. Let the cupcakes cool in the pan just until cool enough to handle, then carefully transfer them to a wire rack and let cool completely.
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To make the blueberry filling: In a small bowl, whisk together the brown sugar, cornstarch, and salt until no lumps remain. In a medium saucepan, combine the blueberries and lemon juice. If using fresh blueberries, add the water; if using frozen, skip it.
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Set the pan over medium heat and cook, stirring occasionally and crushing some of the blueberries, until the berries start to release their juices and the liquids start to simmer, 2 to 4 minutes. (If your mixture looks very dry, add additional water 1 teaspoon at a time.)
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Add the brown sugar mixture, stir to combine, and cook, stirring frequently, until the mixture thickens to a jam-like consistency, 2 to 4 minutes. Transfer to a bowl to cool completely. You should have a heaping 3/4 cup blueberry filling.
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To make the maple buttercream: In the bowl of a stand mixer fitted with the flat beater attachment, beat the butter on medium speed until very light and fluffy. Scrape down the sides of the bowl and add 2 cups (227g) of the confectioners’ sugar, maple syrup, maple flavor, milk, and salt.
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Beat the frosting on low speed until the sugar is incorporated, then increase the speed to medium and continue to beat until light and fluffy. Adjust with additional confectioners’ sugar or milk as needed to make a light and spreadable frosting. For bolder maple flavor, add maple flavor by the drop until you reach the desired flavor. You should have about 1 1/2 cups of frosting.
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To assemble the pancake cupcakes: Use a paring knife or a teaspoon cookie scoop to remove a 1" hole out of the center of each cupcake, reserving the scooped-out top. Spoon a heaping 1 teaspoon blueberry filling into each hole, then return the top, pressing gently to plug the hole (it doesn’t have to fit perfectly).
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To make a flat top with the frosting, follow technique #5 in this blog post: With a large round tip, pipe about 3 tablespoons of frosting onto the center of each cupcake; carefully and swiftly flip the cupcake over onto a parchment-lined baking sheet to flatten the top. Chill for 20 minutes, then flip right-side up. (This gives the cupcakes a flat, pancake-like disk of frosting on top.) Finish the cupcakes by spooning another 1 teaspoon of blueberry filling on top of each frosted cupcake, then drizzle with additional maple syrup, if desired.
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Storage information: Store blueberry pancake cupcakes airtight at room temperature for up to 3 days. You can make the buttercream and blueberry filling up to 4 days before assembling the cupcakes; store them separately in airtight containers in the refrigerator. Bring the buttercream and/or blueberry filling to room temperature before using. Rewhip the tempered buttercream before using.
