Creamy Chocolate Frosting

1. Combine sugar, egg whites, and salt in bowl of stand mixer and set bowl over saucepan filled with 1 inch of barely simmering water. Whisking gently but constantly, heat mixture until slightly thickened, foamy, and registers 150 degrees, 2 to 3 minutes.
2. Fit stand mixer with whisk and beat mixture on medium speed until consistency of shaving cream and slightly cooled, 1 to 2 minutes. Add butter, 1 piece at a time, until smooth and creamy. (Frosting may look curdled after half of butter has been added; it will smooth with additional butter.) Once all butter is added, add cooled melted chocolate and vanilla; mix until combined. Increase speed to medium-high and beat until light, fluffy, and thoroughly combined, about 30 seconds, scraping down beater and sides of bowl with rubber spatula as necessary.

The melted chocolate should be cooled to between 85 and 100 degrees before being added to the frosting. If the frosting seems too soft after adding the chocolate, chill it briefly in the refrigerator and then rewhip it until creamy.

TO MAKE AHEAD: Frosting can be made up to 1 day in advance and refrigerated in an airtight container. When ready to frost, warm frosting briefly in microwave until just slightly softened, 5 to 10 seconds. Once warmed, stir until creamy.

CREAMY MALTED MILK FROSTING
Reduce sugar to 1⁄4 cup, substitute milk chocolate for bittersweet chocolate, and add 1⁄4 cup malted milk powder to frosting with vanilla extract in step 2.

CREAMY VANILLA FROSTING
Omit bittersweet chocolate and increase sugar to 1⁄2 cup. (If final frosting seems too thick, warm mixer bowl briefly over pan filled with 1 inch of simmering water and beat a second time until creamy).

CREAMY PEANUT BUTTER FROSTING
Omit bittersweet chocolate, increase sugar to 1⁄2 cup, and increase salt to 1⁄8 teaspoon. Add 2⁄3 cup creamy peanut butter to frosting with vanilla extract in step 2. Garnish cupcakes with 1⁄2 cup chopped peanuts.

CREAMY BUTTERSCOTCH FROSTING
Substitute dark brown sugar for granulated sugar and increase salt to 1⁄2 teaspoon.