When ready to use, whisk egg and buttermilk in a small bowl. Whisk in melted butter and vanilla extract. Measure out 152 g (or 1.25 cups) of the waffle mix and add to the bowl. Stir until combined. The batter is on the thin side but if the mixture looks too thin, (which only seems to happen to me when I make a double batch) add more of the waffle mix by the tablespoon to the bowl. Stir until it’s the right consistency. Let stand for 30 minutes.