Preheat oven to 350°F. Butter and flour an 8-inch round pan. Beat egg whites until foamy, gradually add ¼ cup sugar, and whip to stiff peaks.

Cream butter with remaining sugar, then beat in yolks one by one and the vanilla.

Whisk flour, baking powder, and salt. Add to butter mixture in three additions, alternating with milk.

Fold a spoonful of whites into the batter, then gently fold in the rest until no streaks remain.

Pour into pan, smooth top, and bake 25–30 minutes until a toothpick has moist crumbs. Cool 10 minutes in pan, turn out, cool completely, then slice and dust with powdered sugar.