Tweed Squares

1 . Stage

Preheat oven to 350 degrees F (175 degrees C). Grease a 9x9-inch square cake pan.

2 . Stage

Beat egg whites in a glass or metal bowl until stiff peaks form. Lift your beater or whisk straight up: the egg whites will form sharp peaks.

3 . Stage

Beat unsalted butter and white sugar together in a large bowl with an electric mixer until light and fluffy; beat in flour, baking powder, and 1 teaspoon salt. Beat milk and 1 teaspoon vanilla extract into flour mixture; fold in beaten egg whites and chocolate chips. Spread batter into prepared pan.

4 . Stage

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 25 minutes. Cool completely in the pan on a wire rack, about 30 minutes.

5 . Stage

Beat 1/3 cup butter, confectioners' sugar, egg yolks, 1 teaspoon vanilla extract, and the pinch of salt together with an electric mixer until smooth; spread on the cooled cake.

6 . Stage

Melt chocolate squares with 2 tablespoons butter in a microwave-safe bowl, about 30 seconds on High. Allow chocolate mixture to cool slightly, then spread chocolate carefully over iced cake. Refrigerate cake until chocolate is set, about 30 minutes.