Melt the butter in a small microwave-safe bowl and then place the bowl in the fridge to allow it to come back to room temperature. In a liquid measuring cup, add 1 tablespoon white vinegar and fill to the 1-cup line with milk. Whisk together and set aside for at least 5 minutes.
In a large bowl, whisk together the flour, cornstarch, baking soda, baking powder, cinnamon, and salt. (Add salt to personal preference and depending on the type of salt you have. I use fine sea salt and add 1/2 teaspoon. If you use table salt, you'll likely want to decrease.) Add in the brown sugar and stir until combined. Make a large well in the center of the dry ingredients.
In another bowl, whisk the 2 eggs and 1 egg yolk with a fork. Pour in the vanilla and heavy cream and whisk to combine. Add the milk and vinegar mixture and whisk until everything is combined.
Pour the wet ingredients into the well of the dry ingredients. Using a wooden spoon, gently stir the wet and dry ingredients until only a few streaks of flour still remain, but most of the ingredients are incorporated (about 10 stir strokes). Pour in the melted (but now to room temperature) butter. Stir until just combined, being careful not to overmix.
Let the batter stand at room temperature for 10 minutes. Preheat the waffle iron (See Note 1). Preheat the oven to 200 degrees F. Place a cooling rack on a large sheet pan and set aside.
Using a 1/3 measuring cup, scoop batter onto the iron and cook according to the waffle maker instructions. If your waffle maker doesn't have a timer indicating when it's done, watch until steam stops coming out the side (usually around 4 and 1/2 to 5 minutes --See Note 2). Remove to the prepared sheet pan with the cooling rack (so your waffles remain crispy and the steam doesn't make them soggy) and place in the 200-degree oven or eat right away with your favorite toppings. Repeat the process of cooking waffles until all the batter has been used.