Preheat your oven to 350°F. Grease a 9-inch square cake pan and line the bottom with parchment paper so your cake lifts out clean. This takes one minute but saves you headache later.
Cream together the butter and sugar until light and fluffy, about 3 minutes with an electric mixer. The mixture should look almost pale and stick to the beaters when you lift them. This aerates the batter and makes your cake tender instead of dense.
Add eggs one at a time, beating after each addition until fully combined. Pour in the vanilla powder and mix for 10 seconds. I always scrape down the sides here because sometimes an egg white hides in the corner and ruins the texture.
In a separate bowl, whisk together flour, baking powder, and salt. Fold the dry mixture into the wet ingredients using a spatula until just combined—don't overmix or you'll get a tough cake. I do about 15 gentle folds, then stop. The batter should look smooth but not overdone.
Pour batter into your prepared pan and smooth the top with an offset spatula. Bake for exactly 25 minutes, until a toothpick inserted in the center comes out with a few moist crumbs. Not completely clean—that means it's overbaked. Let it cool completely on a wire rack.
Once cooled, cut the cake into 3/4-inch cubes using a sharp serrated knife. I always get some wonky pieces and that's totally fine—they still taste perfect. Honestly, the imperfect chunks get hidden in the middle layers anyway, so don't stress about perfect squares.
Now comes the fun part: assembly. Grab a large glass trifle bowl (or a 2-quart clear serving dish if you don't have a traditional one). Layer like this: half the cake cubes on the bottom, then half the whipped cream, then half the red jelly cubes, then half the strawberries and blueberries mixed together.
Repeat the layers exactly the same way—cake, whipped cream, jellies, then berries on top. Chill the red white blue trifle family for at least 1 hour before serving so the layers set and meld together. I serve mine with extra whipped cream on the side because, hello, dessert.