I'm looking for a recipe for coffee frosting, not seven-minute icing or the kind of buttercream that you beat until it's shiny and slippery. I like the basic buttercream frosting in the King Arthur baking book. It's very spreadable and retains just enough texture from the 10X sugar so that it doesn't create the greasy mouthfeel so many of the more slick buttercreams leave behind.
Should I add espresso powder dissolved in water to basic white frosting in place of the milk called for in the recipe? Or would coffee extract be better? If anyone has a recipe for coffee frosting, I'd appreciate it if you'd share it with me.
My plan is to make a chocolate sour cream layer cake with raspberry filling and coffee frosting.