Why has my buttercream turned hard as rocks?

I'm making a chocolate birthday cake with a chocolate buttercream. I'm using a new recipe for buttercream which includes a meringue, granulated sugar, melted chocolate, and espresso powder (notice there's no confectioner's sugar in it). I made it and it tastes DELICIOUS, but when I iced my cake it didn't crust at all. I waited about two hours and then stuck it, covered, in the fridge for the night. I woke up this morning and took the cake out only to find that the buttercream almost appears frozen (but it's not cold enough in the fridge to be frozen). So I'm currently trying to bring it down to room temperature, but can anyone tell me why this happened or how I might be able to fix it?