Reserved drippings from the rib-roast fat add richness to this Yorkshire pudding.
Whisk flour, eggs, 1 1/2 cups milk, salt, and pepper in a medium bowl. Let stand for 30 minutes.
Preheat oven to 400 degrees. Whisk remaining 1 1/2 cups milk into batter. (The batter should resemble heavy cream.) Pour 1 teaspoon drippings into each popover cup (1/2 teaspoon if using muffin tins), then place in oven for 2 minutes. Carefully pour batter into cups, filling each 1/3 full. Bake until puffed and golden brown, about 30 minutes. Serve immediately.