Whisk the flour, milk, salt and eggs together in a bowl. Stir in the butter and whisk until smooth and thin. Cover the bowl with plastic wrap and chill for 6 hours to overnight.
Pour a little vegetable oil in each cup of a popover pan or muffin pan and pre-heat the pan in the oven for 10 minutes.
Pour the batter into the cups of the pre-heated pan and transfer the pan to the oven.
Bake for 45 minutes. DO NOT OPEN THE OVEN during this time.