Step 1

Preheat oven to 325 degrees F. Lightly grease an 8x8-inch baking pan and set aside.

Step 2

In a metal bowl, beat egg whites with a hand mixer on low for 30 seconds, then add 1 tablespoon white sugar and continue beating on high until stiff peaks form. Set aside.

Step 3

In a separate large bowl, use hand mixer to beat egg yolks with 3/4 cup sugar until light and fluffy, about 2-3 minutes. Add vanilla and butter; continue mixing for an additional 2-3 minutes.

Step 4

Fold in flour, cocoa powder, and salt.

Step 5

Slowly beat in milk until completely incorporated.

Step 6

Gently fold egg whites (1/3 at a time) into batter. Note: this will seem extremely difficult to do gently since your egg whites will float around in the batter as you fold. Don't worry: you should still see some white bits in the batter when you're finished. Be sure not to whisk the egg whites, since doing so will deflate your cake.

Step 7

Pour batter into prepared pan and bake for 40-55 minutes in preheated oven, or until custard layer has set and the center of the pan jiggles only slightly when shaken.

Step 8

Serve warm or allow to chill overnight for at least 6 hours before dusting with powdered sugar and serving. Store leftovers in a tightly-sealed container in the refrigerator.