Preheat the oven to 350℉, and line a cookie sheet with parchment paper. Set aside.
In a medium-large mixing bowl, combine the butter and sugar. Mix together well.
ONE tablespoon at a time, add the liquid ingredients and mix well after each addition: 2 tablespoons of milk and 2 tablespoons water. This is essentially creaming the ingredients together.
Stir in the vanilla until combined.
On top of the cookie dough, add the dry ingredients: flour, baking powder, baking soda, sea salt, and cornstarch. Stir into the batter gently, then add the sprinkles mixing just until combined.
Using a one tablespoon cookie scoop, spoon onto lined cookie sheet, about 2 inches apart. Bake 12-14 minutes. Allow cookies to set for a few minutes by leaving them on the pan after baking until they have cooled slightly.