Tweed Squares - cooking recipe

Ingredients

Cake:
2 egg whites
1/2 cup unsalted butter
2/3 cup white sugar
1 1/3 cups all-purpose flour
2 teaspoons baking powder
1 teaspoon salt
1/2 cup whole milk
1 teaspoon vanilla extract
1/2 cup miniature semisweet chocolate chips
Frosting:
1/3 cup butter, softened
1 1/2 cups confectioners' sugar
2 egg yolks
1 teaspoon vanilla extract
1 pinch salt
Topping:
3 (1 ounce) squares semisweet chocolate
2 tablespoons butter

Preparation

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