Preheat oven to 350 degrees F. Butter 3 9-inch round baking pans and line the bottom of each with a round of parchment paper. Butter parchment paper and lightly dust pan with flour, shaking out excess.
Sift cake flour into a mixing bowl. Add all-purpose flour, baking powder, baking soda and salt and whisk for 30 seconds, set aside.
In the bowl of an electric stand mixer, fitted with the paddle attachment (if you don’t have the paddle attachment that constantly scrapes sides of the bowl while mixing, then stop mixer occasionally throughout entire mixing process and scrape down sides and bottom of bowl), whip together butter, granulated sugar and lemon zest until mixture is pale and fluffy.
In a 2 cup liquid measuring cup, measure out milk then stir in sour cream and lemon juice, let rest 3 minutes.
Meanwhile, mix eggs into butter mixture one a time, then stir in lemon extract and vanilla extract. Toss blueberries with 3 Tbsp of the flour mixture.
Working in three separate batches, beginning and ending with remaining flour mixture, add 1/3 of the flour mixture (to the butter/egg mixture) alternating with 1/2 of the milk mixture and mixing just until combined after each addition. Gently fold in blueberries.
Divide mixture evenly among prepared baking pans. Bake in preheated oven until toothpick inserted into center comes out clean, about 24 – 27 minutes.
Cool in baking pans about 20 – 30 minute then invert onto wire racks to cool completely. Once cool, frost with cream cheese frosting and decorate with blueberries and lemon slices if desired. Store in an airtight container.
Make Cream Cheese Frosting
In the bowl of an electric stand mixer fitter with the paddle attachment, whip butter until pale and fluffy.
Add cream cheese and mix until smooth and fluffy. Add vanilla and lemon extract and powdered sugar and blend several minutes longer until smooth and fluffy (if frosting seems slightly runny, you can chill it for a bit before frosting cake).
You need to whip the butter and sugar together until the mixture is very pale, almost white. If you don’t whip the butter enough, the cake will turn out dense and heavy.
*Omit lemon extract if making this cream cheese frosting for a different recipe. This frosting is good on so many different kinds of cake!