I use this recipe with a couple of tweaks.
12 egg whites
1 and 1/2 teaspoons cream tartar (or some fresh lemon juice)
1 and 3/4 cups (350 grams) granulated sugar
Splash of vanilla extract
1 cup + 2 tablespoons (133 grams) cake flour
Pinch of salt
Preheat oven to 325°F (160°C).
In a mixer with a whisk attachment, whip the egg whites and cream tartar on low speed at first, increasing the speed after a couple of minutes. Slowly add 100 grams (0.5 cups) of the fine sugar in “rain” form. Add the vanilla extract and mix for 1 more minute. You want to get soft peaks, not meringue.
In 4-5 additions, sift the flour, salt, and the rest of the sugar into the egg whites, gently folding with a rubber spatula.
Pour the batter into an ungreased 9-inch pan or a muffin pan. It’ll make around 15 normal-sized muffins.
Bake until a toothpick comes out clean. It’s around 20 minutes for muffins and 45 minutes for a cake.
To cool down the cake, flip it upside-down on a wire rack for a few hours, until completely cool. Running a knife around the edges will release the cake.
Note: whipped heavy cream and berries would be perfect toppings.