Preheat. Heat oven to 425F. Place a standard 12-cup muffin tin in the oven while it preheats.
Make the batter. In a blender, add eggs, milk, flour, sugar (if using), salt, and vanilla (if using). Blend 20 to 30 seconds until smooth. (Or whisk very well in a bowl.)
Butter the tin. Carefully remove hot muffin tin. Add about 1 teaspoon butter to each cup (use the 4 tablespoons total). Return to oven for 2 minutes until butter is melted and bubbling.
Fill. Remove tin again and quickly pour batter evenly into cups (about 3 tablespoons each).
Bake. Bake 12 to 15 minutes until puffed and deeply golden around the edges.
Serve. Serve immediately with powdered sugar, berries, syrup, and lemon if you like.