I rarely make buttercream frosting – maybe once every three or four years. As a consequence I pull out half a dozen main references to crosscheck, follow them, and make the same stupid mistake every time. So in the hopes that next time I’ll remember to just check my notes:
by weight: one part fat (classically butter, can be other solid fats) to two parts confectioner’s sugar. NOT four parts confectioner’s sugar.
Too much sugar makes a stiff, slightly gritty mass that doesn’t particularly stick to anything. Keeping the sugar down and whipping it at the finish means I can substitute the fluff for the stiff.
Cream the butter. Cream in the sugar. Whip at finish.
As ‘by the way’ (because I made more than one batch yesterday), high-fat (60% or better) chocolate can substitute for butter at pretty close to 1:1 – close enough for my purposes anyway. But don’t go more than 1/2 as chocolate.
Maybe next time I can have it turn out right the first time instead of tossing butter and sugar in the trash.