Preheat your oven to 350°F and line a 9x13-inch baking pan with parchment paper. I always do this while gathering ingredients because cold ovens are the enemy of even rise—the batter waits less than 10 minutes total.

Cream together the butter and sugar until the mixture resembles wet sand, about 3 minutes with an electric mixer. This step matters because air pockets form here, and they're what makes the cake feel light instead of dense throughout baking.

Add eggs one at a time, beating 20 seconds between each addition. I skip the temptation to dump all three at once because it prevents the emulsion from breaking—honestly, patience here saves texture problems later.

Whisk together flour, baking powder, and salt in a separate bowl before adding to the wet mixture. Alternating the dry ingredients with milk (flour, half the milk, remaining flour, rest of milk) prevents lumps and keeps the batter smooth.

Gently fold in the strawberry and blueberry purees with a spatula, counting to 60 and stopping the moment you don't see streaks anymore. Overworking here toughens the cake and can make it sink in the middle during baking.

Pour batter into your prepared pan and smooth the top with an offset spatula. Bake for 45 minutes until a toothpick inserted in the center comes out with just a few moist crumbs, not completely clean.

Cool the cake completely in the pan before turning it out—at least 2 hours, or overnight works too. This prevents the warm cake from falling apart when you flip it, which Connor learned the hard way last year.

While cake cools, whip heavy cream with powdered sugar until stiff peaks form, then spread evenly across the top surface, creating a white stripe down the center.