In a large bowl, mix together flour, sugar, baking powder, baking soda, and salt.
In another bowl, whisk together 1 1/2 cups milk, eggs, and vanilla. Stir into dry mixture to form a batter.
Scoop about half the batter into another bowl. Stir in remaining 1/2 cup milk, then add cocoa powder and espresso powder (if using).
Preheat waffle iron.
Divide melted butter between two batters and mix in. Once waffle iron is preheated, spray with nonstick spray (if desired). Using two scoops, pour both vanilla and chocolate batters into waffle iron at the same time. Cook according to manufacturer's instructions and repeat until batter is gone.
Serve waffles with whipped cream, sprinkles, syrup, or fruit. Reheat any leftover waffles in the oven or toaster.