Flag Layer Cake

Preheat oven to 350 degrees. Butter two 8-by-2-inch round cake pans; line with parchment rounds and butter parchment. Whisk together flour, baking powder, baking soda, and salt in a medium bowl.

With a mixer on medium speed, beat butter until softened, 2 minutes. Add sugar; continue beating until pale and fluffy, 4 minutes. Add eggs, one at a time, beating well after each addition. Beat in vanilla.

Reduce speed to low and slowly add flour mixture in 2 batches, alternating with buttermilk and beginning and ending with flour mixture; beat just until incorporated.

Divide batter evenly between prepared pans. Bake, rotating halfway through, until cakes are golden brown and a tester inserted into centers comes out clean, 35 to 45 minutes. Transfer pans to a wire rack; let cool 15 minutes. Turn out cakes onto racks and let cool completely.

In a bowl, combine strawberries with strawberry jam. In another bowl, combine blueberries with blueberry jam.

With a large serrated knife, trim cake tops level; split each cake in half horizontally. Fill a small pastry bag with 1/2 cup buttercream; cut a 1/4-inch tip. Transfer one layer to a cake stand or platter. Pipe a circle of buttercream around outside of cake (to keep blueberry mixture from spreading out), then spread blueberry mixture evenly inside circle. Place another cake layer on top; spread evenly with 1 cup buttercream. Place a third cake layer on top; pipe a circle of buttercream around outside of the cake, then spread strawberry mixture evenly inside circle. Place remaining cake layer on top. Spread buttercream over top and sides of cake to form a crumb coat; refrigerate until firm, about 30 minutes. Spread remaining buttercream over top and sides of cake, using an offset spatula to smooth sides and create swirls on top.