Beat 1 1/4 cups dark brown sugar and 1 stick butter until fluffy, then beat in 1 egg and 1 teaspoon vanilla. Whisk 1 1/2 cups flour, 1/4 cup malted milk powder, 1/2 cup Dutch-process cocoa powder, 3/4 teaspoon salt and 3/4 teaspoon baking soda; stir into the butter mixture. Chill 30 minutes, then roll into balls. Press a halved malted milk ball into the center of each cookie before baking, 12 to 15 minutes at 350 degrees.
Recipe courtesy Food Network Magazine