Preheat oven to 450 F. Position oven rack in the bottom of the oven (to reduce risk of over-browning).

Mix flours (spoon and level to measure--you don't want a heavy batter), salt and sugar in a bowl and set aside.

Heat the milk in a small saucepan until just warm to the touch.

Blend milk, eggs and vanilla in a blender until frothy.

Add flour and blend until well combined. Add butter and blend again.

Spray popover (or muffin pan--see note) pan with non-stick spray and fill cups about 3/4 full.

Bake on lowest oven rack position for 20 minutes at 450F, then lower heat to 350. Bake an additional 10-20 minutes until brown and very set (may vary by popover size). Do not open the oven (unless they are going to burn, then slide a cookie sheet into a rack above and start at 425 F next try).

Remove from oven and take out of pan immediately. Serve warm.