In a blender or food processor, whirl eggs, all-purpose and buckwheat flours, and milk until smooth, scraping down sides of container as necessary. The buckwheat flour may settle as batter stands; stir before cooking if necessary.
Set a nonstick frying pan 8 inches across bottom over medium-high heat; when hot, brush bottom with butter, then wipe out excess with a paper towel.
Lift pan from heat and pour in 1/4 cup batter; immediately tilt pan and swirl batter to coat bottom. Crêpe should set at once and form tiny bubbles. Set pan back over heat and cook crêpe until lightly browned at the edge and dry-looking on the surface, 1 to 3 minutes.
Run a wide spatula under crêpe edge to loosen. Turn crêpe over and fill immediately.