Heat oven to 350'F. In small saucepan over very low heat, melt peanut butter chips and 3 tablespoons softened butter. Remove from heat; cool slightly. In large mixer bowl, beat remaining 1/2 cup softened butter and 1 cup sugar until light and fluffy. Add corn syrup, egg and vanilla; blend thoroughly. Combine 2 cups flour, baking soda and salt; add to butter mixture, blending well. Remove 1 1/4 cups batter and place in a small bowl; with wooden spoon, stir in remaining 2 tablespoons flour and peanut butter chip mixture. Blend cocoa, remaining 1/4 cup sugar and melted butter into remaining batter. Refrigerate both batters 5-10 minutes or until firm enough to handle. Roll both doughs into 1" balls; roll in additional sugar. Place on ungreased cookie sheet. Bake 10-11 minutes or until set. Cool slightly; remove from cookie sheet to wire rack. Cool completely. Place 1 marshmallow on flat side of 1 chocolate cookie. Microwave at MEDIUM (50%) 10 seconds or until marshmallow is softened; place a peanut butter cookie over marshmallow, pressing down slightly. Repeat with remaining marshmallows and cookies. Serve immediately. Makes about 2 dozen sandwich cookies.