I’ve received a couple of questions on the subject this week. Honestly I didn’t know single-acting powder was still out there for anything other than commercial/industrial use. As I mentioned below, double acting baking powders deliver two “actions”, one that happens fast when the batter is being mixed, and another that happens slowly as the batter gets hot. The effect is achieved by combining two different acids with a specific quantity of baking soda (plus a little corn starch to keep the chemicals from reacting in the can). For the fast reaction baking powder manufacturers usually use either cream of tartar or mono calcium phosphate (MCP). For the slow one, typically either sodium acid pyrophosphate (SAPP), sodium aluminum phosphate (SALP) or sodium aluminum sulfate (SAS), or a combination.
As you’ve probably deduced by now, single-acting baking powders deliver only one of those two actions: either the fast one or the slow one. You can tell which by checking the ingredient label. My guess is that anything sold at the retail level would probably be the slow-acting kind, designed to be used in combination with the creaming method. I’m not sure what good a fast-reacting single-action powder would do a home baker to be honest. But then it’s early in the morning and I haven’t had any caffeine yet.