Sift together cake flour, cream of tartar, and salt in a small bowl. Do this three times total — I know it seems like overkill, but it removes lumps and aerates the flour, which actually matters here. Set the mixture aside while you prep your egg whites.

Add 6 egg whites to a super clean mixing bowl (no grease at all or they won't whip). Use an electric mixer on medium speed and beat until soft peaks form, about 3 minutes. Soft peaks mean the whites hold a shape but the tips curl down.

Gradually add 1 1/2 cups granulated sugar, a few tablespoons at a time, while beating on medium-high speed. This takes about 5-7 minutes total, so don't rush it. You'll know you're done when the whites look glossy and form stiff peaks that stand straight up.

Gently fold in the vanilla bean paste with a rubber spatula using about 15 careful strokes. Now fold in the flour mixture in 4 additions, folding just until combined each time — overfolding deflates all those air bubbles you worked hard to create.

Pour the batter into an ungreased 10-inch tube pan. Yes, ungreased — the batter needs to grip the sides or it won't rise properly. Bake at 350°F for 40 minutes until golden brown and a toothpick inserted in the center comes out clean.

Remove the cake from the oven and immediately invert it onto a cooling rack or bottle neck (this stops it from deflating). Let it cool completely upside down, about 2 hours — patience here is everything. Once cool, run a thin knife around the edges and center tube to loosen it.

For the topping, whip 1 cup heavy cream with 2 tablespoons powdered sugar and 1 tablespoon lemon juice until stiff peaks form, about 3 minutes. Carefully turn the cake right-side up, slice into 12 wedges, and top each slice with whipped cream, fresh strawberries, blueberries, and a tiny pinch of chopped mint.