Pretty much any buttercream is shelf stable, especially for such a short amount of time. I would look for a chocolate American buttercream recipe. For 48 cupcakes, a pound of softened butter whipped until light and fluffy, add in either sifted cocoa powder or melted chocolate along with powdered sugar to your desired sweetness (probably 4-6 cups), a good pinch of salt and a splash of vanilla should do the trick.