Pre-heat the waffle maker.

Combine the flour, cornstarch, salt and baking powder in a bowl.

Combine the egg yolks, buttermilk, milk, vegetable oil and vanilla extract (if using) in a second bowl or measuring cup.

In a third bowl, whisk the egg whites to soft peak stage.

Pour the wet ingredients into the dry ingredients and stir just until combined, but still a little lumpy. Fold in the egg whites until no streaks of white remain.

Grease your waffle iron if necessary and pour ½ cup of waffle batter onto the waffle maker, spreading it out to cover the plate. Place two slices of ham on top of the batter. Add a few leaves of spinach and sprinkle ½ cup of the Gruyère cheese on top. Pour another ½ cup of the waffle batter over the cheese and spread it evenly so it completely (or almost) covers the spinach. Close the lid tightly. (You will want to work quickly when building the waffle so the bottom side does not get too brown.)

After about 5 minutes, remove the waffle from the iron and keep it warm directly on the oven rack in a 200ºF oven until you’re ready to serve.

Combine the Dijon mustard and honey to serve with the waffles as a dip.