Heat oven to 425°. Whisk together flour, salt add milk till barely combined; Mixture will be lumpy.
Beat in eggs and yolk one at a time until barely incorporated.Batter should still be lumpy.
Brush interior of popover pan generously with butter. Divide batter evenly among tins. Bake 20 mins.
Reduce oven to 350°; bake until puffed and golden brown, 15-20 mins more. Serve immediately or pop to let steam escape and hold in 200°oven up to 30 mins more.