Use your finger, or a spatula, to rub lemon zest into the sugar in a large bowl.
Whisk in flour, cornstarch, baking powder, baking soda, and salt.
In another bowl whisk buttermilk, milk, and ricotta until smooth.
Add melted butter, oil, lemon juice, and eggs. Whisk well.
Stir wet ingredients into dry until mostly combined.
Stir in blueberries and let batter rest 15 minutes. Save some blueberries for garnishing the top of your waffles.
Dollop blueberry jam on top and swirl gently. Save some blueberry jam for serving.
Cook batter in a preheated waffle maker for about 4 to 5 minutes, or until your waffle maker says they are done..
Keep waffles warm in a 200°F oven until serving.