Blueberry Sheet Pan Pancakes

Preheat the oven to 450°F and grease a half-sheet pan with melted butter or neutral vegetable oil. Be sure the grease the sides, too.
Whisk together the milk, buttermilk, egg, and vanilla extract. Whisk a few tablespoons of the milk mixture into the melted butter to temper, and then pour the tempered butter into the liquid. Whisk to fully combine.
In a large mixing bowl, combine the flour, baking powder, baking soda, salt, and sugar. Add the liquid to the dry ingredients and use a spatula to stir, just until combined. It's ok if there are a few lumps.
Place the greased sheet pan in the oven and heat for a solid 5 minutes while the pancake batter stands and you pour yourself a fresh cup of coffee.
Taking care not to burn yourself, remove the hot pan from the oven and place it on a cooling rack or trivet. Pour the pancake batter into the sheet pan and spread evenly. Sprinkle the blueberries or any toppings of choice all over.
Bake for 13 to 15 minutes, until the pancake is baked through and the center springs back when lightly pressed, like a cake.
Change the oven setting to broil. Place the pancake under the broiler and brown the top. This will take around 1 or 2 minutes, but check almost constantly; it will brown very fast.
Cut into squares and serve hot with butter and syrup.