Beat whipped quinoa with sugar, vanilla and salt on high speed until the sugar is dissolved, about 5 minutes. With mixer running, add butter, a few pieces at a time, and continue beating until all the butter is incorporated and frosting is light and fluffy, 5–7 minutes. Spread or pipe frosting onto cooled cupcakes. Note: If you refrigerate the frosting, let it come to room temperature, then beat it again just to fluff it up.