Turn the mixer off and use a rubber spatula to make sure that all of the flour has blended in with the liquid. Set the mixer to medium speed and dribble in the remaining milk. Blend for 1 minute. Extra beating can't hurt. Bake at for 15 minutes.
Notes Serve immediately, with fresh jam and room-temperature butter.
These popovers turn out significantly better if they are baked in crockery custard cups rather than in metal or glass.