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Make cake: Preheat oven to 325°F. Set out a 5x3-inch round pan. Do NOT grease the pan. Place the pan on a sheet pan (just in case any batter spills out as it bakes).
If adding the optional sugar, whisk together cake flour and 3 Tbsp sugar in a small bowl. Set aside.
Using a hand mixer or in the bowl of a stand mixer with the whisk attachment, combine egg whites, cream of tartar, and vanilla. Beat on medium-high speed until the egg whites form soft, fluffy peaks, 3 to 4 minutes. (Note: This is the most important step in making this cake. Beat the egg whites until the entire mixture is opaque, white, and fluffy throughout. The peaks should stand up straight with a little bit of a curl at the top when you lift the beaters out of the mixture.)
Sprinkle the cake flour over the top of the egg whites. Use a whisk or spatula to gently fold the flour mixture in, being careful not to compress the fluffy egg whites.
Pour the batter into the springform pan. (Note: Fill the pan right up to the top, rounding it gently with a spatula to create a smooth surface.)
Bake until cake is set and the top is a light golden brown, 20 minutes.
Remove cake from oven and invert over a cooling rack or two glasses / cans. Allow cake to cool completely upside down (for at least 1 hour).
Run a knife around the edges of the cake to gently release it from the pan.
Make whipped cream: Using a hand mixer or in the bowl of a stand mixer, combine cream with 2 tsp sugar (if using). Beat cream on medium-high until light, fluffy, and firm enough to spread over the top of a cake, 3 to 5 minutes.
Just before serving (and when the cake is completely cool), spoon whipped cream over top of cake. (Note: If you want an even more photo-friendly topping, you can transfer the whipped cream to a small ziptop / plastic bag. Seal the bag and then cut off one corner. Genty squeeze the whipped cream out over the top of the cake in billowy rounds.)
Top the cake with fresh berries, if you'd like. Serve!