To make brown butter: Melt butter in a medium saucepan, preferably not non-stick, set over medium. When butter becomes foamy, stir often until it smells nutty and turns dark brown, about 4 min. Immediately remove from heat and pour into a small bowl. Use right away or refrigerate up to a week. To melt brown butter, microwave on high for 15 to 30 sec.
To make cupcakes: Whip up a batch of cupcakes, using either our chocolate cupcake recipe or chocolate-cake mix. For icing: Refrigerate brown butter until it has the texture of room temperature butter, about 20 min. Beat equal amounts brown butter and icing sugar, using an electric mixer, until buttercream is fluffy and pale.