In a large bowl, blend sugars, peanut butter, butter, shortening and egg. Stir in remaining ingredients. Cover and chill for 2 hours, or until firm. Form dough into one-inch balls; place 1 1/2 inches apart on ungreased baking sheets. Make an indention in each ball with your thumb. Bake at 375° for 9 to 10 minutes, or until lightly golden. Remove the pan from the oven and press a chocolate drop into the center of each. Cool two minutes on baking sheets. Remove to wire rack to cool completely. Makes 2 1/2 to 3 dozen.