Preheat oven to 325°F (160°C). Butter a 9-inch springform pan or line an 8x8-inch baking dish. In a bowl, whip egg whites until foamy, then gradually add 1/2 cup sugar and beat to stiff, glossy peaks.

In a large bowl, whisk yolks with remaining 1/4 cup sugar until pale, about 2 minutes. Whisk in melted butter and vanilla until smooth.

Sift flour and salt into yolk mixture. Stir to combine, then gradually whisk in lukewarm milk until batter is thin and smooth.

Stir about 1/3 of the egg whites into the batter to lighten it. Gently fold in remaining whites in two additions, leaving a few small streaks.

Pour batter into prepared pan, tap once on the counter, and bake 50-55 minutes until top is golden and springs back; center will jiggle.

Cool completely in the pan on a wire rack. Dust with powdered sugar, slice, and serve.