Preheat your oven to 425 degrees F. Place a 12-cup muffin tin on the center rack to heat up while you prepare the batter. This preheating step ensures maximum puff.
In a blender, combine flour, milk, eggs, sugar, vanilla extract, and salt. Blend on medium speed for 30 seconds until completely smooth with no lumps visible. The batter should appear thin like crepe batter.
Carefully remove the hot muffin tin from the oven using oven mitts. Working quickly, add about 1 teaspoon of melted butter to each cup, swirling to coat the bottom and sides.
Immediately pour the batter into each cup, filling about two-thirds full. Return the tin to the oven right away to maintain temperature.
Bake for 15 to 18 minutes without opening the oven door. The mini pancakes will puff dramatically and turn golden brown. Centers should look set but still slightly jiggly.
Remove from oven and serve immediately while still puffed. Run a butter knife around edges to release each mini pancake. They will deflate quickly but maintain their delicious texture.
Top with your favorite toppings such as powdered sugar, maple syrup, fresh berries, or whipped cream. Serve hot and enjoy the light, custardy texture.