1. Preheat oven to 350 degrees F (175 degrees C). Grease a 9x9-inch square cake pan. 2. 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. 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. 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. 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. 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. ---------------------------------------------------------------------------
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