In a medium light-colored metal saucepan, cook butter over medium heat, stirring frequently, until butter solids are golden and nutty in aroma, 7 to 12 minutes. Pour browned butter into a square glass baking dish or medium bowl; refrigerate until solid, 2 to 3 hours. Let stand at room temperature until softened before using, 30 to 40 minutes.

In the bowl of a stand mixer fitted with the paddle attachment, beat softened browned butter, sugar, and salt at medium speed until fluffy, about 3 minutes. Use immediately.