What is creaming method?

A mixing technique where solid fat (butter, usually softened or in the brownie case re-solidified brown butter) is beaten with granulated sugar until the mixture lightens in color and grows in volume — typically 3-5 minutes in a stand mixer with the paddle attachment, or 5-7 minutes by hand. The mechanical action forces air pockets into the fat-sugar mass; those pockets are the foundation of leavening in many cake and cookie recipes. Brown sugar is added AFTER the creaming step (rather than with the granulated sugar) because the molasses content interferes with the air-incorporation process. The creaming step is what separates the brown-butter brownie method from the simpler melt-and-whisk approach — you trade time for a deeper crackle on top and a slightly lighter crumb.