Whisk together milk and mayonnaise until smooth. Stir milk mixture into flour just until blended.
Butter a 10-inch cast-iron skillet; place in a 425° oven for 3 minutes.
Remove skillet from oven; drop batter by rounded 1/4 cupfuls into hot skillet, dropping 8 around edge and 4 in center. (Biscuits will touch.)
Bake at 425° for 25 to 30 minutes or until lightly browned.