This recipe highlights berries twofold, because not only is it beautifully decorated in fresh berries, but it has a sweet and thick filling made from fresh blueberries and lemon juice.
Preheat oven to 350 degrees. Lightly grease bottoms and sides of two 9-inch round cake pans. Line bottoms of each pan with a round of parchment paper. Grease paper and dust each pan with flour, tapping out excess. Set aside.
In a large bowl, whisk together flour, baking powder, and salt. Set aside. In a small bowl or liquid measuring cup, combine buttermilk, lemon zest, and vanilla. Set aside. In the bowl of an electric mixer, beat butter and sugar on medium-high speed 5 minutes or until light and fluffy. Add eggs and yolks, one at a time, with mixer on low speed, scraping down sides of bowl as necessary. Alternately add flour mixture and buttermilk mixture in several additions, starting and ending with flour mixture, mixing just until combined.
Spread batter into cake pans and bake 35 minutes or until golden and cake tester or toothpick inserted into center of cakes comes out clean. Cool on wire racks 10 minutes. Invert cakes onto wire racks. Remove and discard parchment paper and let cake layers cool completely.
In a medium saucepan, bring blueberries, lemon juice, sugar, and lemon zest to a boil. Reduce to a simmer and cook about 15 minutes or until blueberries have burst and mixture thickens, stirring frequently. In a small cup, combine water and cornstarch. Stir cornstarch mixture into blueberry mixture and return to a boil. Boil 1 minute, stirring constantly. Remove from heat and let cool completely (refrigerate if necessary).
Beat the cream cheese, butter, and vanilla with an electric mixer on medium-high speed until blended. Add confectioners' sugar and mix on low until combined.
Place one cake layer on a cake plate or stand. If cakes have rounded top, gently trim with serrated knife to make flat. Spread cooled blueberry filling on cake layer. Place other cake layer on filling, with flat side up and press down gently to secure. Frost top and sides of cake with frosting. Using blueberries, create large star on top of cake. Fill in remaining white space with raspberries. Line bottom rim of cake with blueberries, pressing gently into frosting to secure. If not serving cake immediately, refrigerate until ready.