Preheat oven to 425°F. Place a muffin tin in the oven while it heats.
In a blender or bowl, blend eggs, milk, flour, sugar, vanilla, and salt until smooth and thin.
Carefully remove hot muffin tin and add about ½ teaspoon butter to each cup.
Return pan to oven for 1–2 minutes until butter is melted and bubbling.
Quickly pour batter into each cup, filling about halfway.
Bake 12–15 minutes until puffed, golden, and dramatic.
Remove from oven and serve immediately as they gently deflate.
