These Yorkshire puddings make a great side to a holiday brisket.
In a large bowl, whisk the flour and 1 1/4 tsp. salt. Create a well in the center of the flour mixture. Pour the eggs into the well; whisk until the batter just comes together. Slowly drizzle in the milk, whisking constantly, until a smooth batter forms. Whisk in the horseradish. Cover and let the batter rest at room temperature for at least 1 hour, or chill overnight.
Preheat the oven to 450°. Place a piece of butter in each cup of a 12-cup muffin tin. Heat the pan in the oven until the butter melts and bubbles, about 2 minutes.
Using pot holders, carefully remove the muffin tin from the oven. Stir the batter, then quickly pour it into the tin, filling each cup halfway. Bake until puffed and deeply golden brown at the edges, 15 to 20 minutes. Before serving, sprinkle generously with pepper. Run a knife around each cup to loosen. Serve hot.