Preheat the oven to 220C°/gas mark 7.
1) Generously grease aluminium popover tins or custard cups with softened butter. You'll need enough pans to make 12 popovers. Place the tins in the oven for 2 minutes to preheat.
2) Whisk together the flour, salt, thyme, eggs, milk, and melted butter until smooth. The batter will be thin. Fill the popover tins less than half full and bake for exactly 30 minutes. Do not open the oven door.