This blackberry cake stars a velvety butter cake that sandwiches sweet-tart blackberry jam and softly whipped cream for a decadent, summery cake that's welcome anytime. Decorate the top with fresh blackberries in season.
Heat oven to 350℉. Butter the bottom of an 8-inch round cake pan and line with parchment paper. Butter and flour paper and sides. In a medium bowl, whisk together flour, baking powder, and salt; set aside.
Using an electric mixer on high speed, beat butter and sugar in a large bowl until light and fluffy. Add eggs and yolks, one at a time, beating well after each addition. Beat in vanilla. With mixer on low, alternately add flour mixture in 3 parts and milk in 2, beginning and ending with flour mixture; mix just until combined. Spread batter in prepared pan.
Bake until a toothpick inserted in center of cake comes out clean, 30 to 35 minutes. Cool in pan 10 minutes; invert onto a wire rack to cool completely.
Using a serrated knife, split cake in three layers horizontally; place one layer, cut side up, on a serving plate. Spread generously with blackberry jam and a medium-thick layer of whipped cream. Repeat with the second layer. Top with the final layer and use the remaining whipped cream to cover the top and sides of the cake. Keep refrigerated until ready to serve and refrigerate the leftovers.