In a medium-sized microwave-safe bowl, or in a saucepan set over low heat, melt the butter, then add the sugar and stir to combine. Or simply combine the butter and sugar, and heat, stirring, until the butter is melted. Continue to heat (or microwave) briefly, just until the mixture is hot (about 110°F to 120°F), but not bubbling; it’ll become shiny looking as you stir it. Heating the mixture to this point will dissolve more of the sugar, which will help produce a shiny top crust on your brownies