Combine dry ingredients in a bowl, whisk to combine. Dig hole in center of dry ingredients. Pour in wet ingredients except for coffee. Whisk these ingredients together until smooth. Add coffee, whisk well. Batter will be thin. Bake in 350 degree oven for 25 minutes for a 12 x 17 sheet pan or 35-40 minutes for a 9x13 pan.
Cool completely, and frost. Top with sparkling sugar sprinkles if desired.
In a glass bowl suspended (but not touching) over simmering water, whisk egg whites and sugar until mixture reaches 160 degrees. Transfer marshmallow-like mixture to stand mixer fitted with whisk attachment. Add vanilla and beat hot mixture on high speed until cool (8-10 minutes or so). Once cooled, leave mixer on high and add butter, 1 TBSP at a time until incorporated and fluffy. (This frosting will look curdled before it comes together again, don't worry.) Add melted/cooled chocolate and beat 1 minute more. Use immediately.