Preheat the oven to 350 degrees F (175 degrees C). Grease the cups of a muffin tin.
In a large bowl, stir together the flour, sugar, baking powder, and salt. Pour in the beer and stir until evenly moistened. The dough will be slightly sticky. Stir in peanuts. Fill muffin cups 3/4 full with the batter and spoon about 1/2 teaspoon of butter over the top of each one.
Bake for 25 minutes in the preheated oven, or until browned and firm. Cool in the pan for about 5 minutes before removing and cooling on a wire rack.