Stir together 1 cup peanut butter, 1 cup sugar, 1 large egg, and 1 tsp. vanilla extract in a large bowl until combined. Divide dough in half. Stir 2 melted semisweet chocolate baking squares into half of dough. Shape doughs into 30 (1-inch) half peanut butter, half chocolate-peanut butter balls. Place balls 1 inch apart on ungreased baking sheets; flatten gently with a spoon. Bake at 325° for 15 minutes or until golden brown. Remove to wire racks to cool.
Variations: Evenly press 1 cup of your desired addition, such as chocolate morsels, chocolate-coated toffee bits, or chopped peanuts, onto the top of prepared cookie dough on baking sheets; bake as directed.