Foolproof Sponge Cake
1/2 cup cake flour (2 oz)
1/4 cup all-purpose flour (1.25 oz)
1 teaspoon baking powder
1/4 teaspoon sea salt
3 tablespoons milk
2 tablespoons unsalted butter
1/2 teaspoon vanilla extract
5 large eggs, at room temperature
3/4 cup granulated sugar (can probably decrease to 2/3 cup if you dislike sugar)
1/4 teaspoon cream of tartar
Adjust an oven rack to the lower-middle position and heat the oven to 350°F (175°C). Grease two 8-or-9-inch cake pans and cover pan bottoms with rounds of parchment paper or use paper baking cups.
Whisk/sieve the flours, baking powder, and salt together in a medium bowl and set aside. Heat the milk and butter in microwave on low power just until the butter melts. Add vanilla extract; keep the mixture covered and warm.
Separate the eggs, placing the whites in a large, oil-free metal mixing bowl and reserving the yolks in a small bowl. With hand-held electric whisk, beat the whites on medium-low speed until foamy. Gradually add 6 tablespoons of the sugar and the cream of tartar and continue to beat the whites to soft, moist peaks.
Beat the egg yolks with the remaining 6 tablespoons sugar on medium-high speed until the mixture is very thick and turns a pale lemon color, about 5 minutes. Scrape beaten yolks onto the whites, but do not mix.
Sprinkle/sieve the flour mixture over the egg mixtures and fold gently with a large rubber spatula until barely combined, 12 strokes if you're lucky. Make a well in one side of the batter and pour the melted butter mixture into the bowl. Fold gently with a large rubber spatula until the batter shows no trace of flour and the whites and yolks are evenly mixed, about 8 strokes. Also make sure that you have incorporated the butter into the mixture. There should not be visible grease/oil as you pour the mixture into the cake pans.
Immediately pour the batter into the prepared pans or scoop into the paper cups. Bake until the cake tops are light brown and feel firm and spring back when touched, about 16 to 18 minutes for 9-inch cake pans and 20 to 22 minutes for 8-inch cake pans.
Cool completely on racks. Run a thin knife around the inside of the cake pans and then invert them onto the racks (or onto cardboard rounds or tart pan bottoms) to release the cakes from the pans. Remove the parchment paper.