Preheat oven to 350°F. Lightly grease bottoms and sides of 2 (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.
Whisk together flour, baking powder and salt into a large bowl. Set aside. Combine buttermilk, lemon zest and vanilla in a small bowl or liquid measuring cup. Set aside. Beat butter and sugar in the bowl of an electric mixer 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.
Bring blueberries, lemon juice, sugar and lemon zest to a boil in a medium saucepan. Reduce to a simmer and cook about 15 minutes or until blueberries have burst and mixture thickens, stirring frequently. Combine water and cornstarch in a small cup. 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 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.