Heat oven to 425°F. Grease bottoms of 24 regular-size muffin cups with butter. Unroll pie crusts. Using 2 1/2-inch round cookie cutter, cut 12 rounds from each pie crust, rerolling dough if necessary. Press 1 round in bottom and halfway up side of each cup. In small bowl, beat egg and water until well blended; brush on crusts.