1. Heat oven to 350°F. Grease 13x9x2-inch baking pan.
2. Melt butter in medium saucepan over low heat. Add 2 cups sugar and cocoa; stir to blend. Remove from heat. Stir in eggs and vanilla. Stir together flour, salt and baking soda; stir into chocolate mixture. Stir in 2/3 cup mini peanut butter cups; spread mixture in prepared pan.
3. Combine peanut butter and remaining ¼ cup sugar in microwave-safe bowl.
Microwave at HIGH (100%) 1 minute or until fluid. Drop by tablespoons onto chocolate batter. Swirl with metal spatula or knife for marbled effect.
4. Bake 25 to 30 minutes or until brownie starts to pull from edges of pan and is set. Cool
45 minutes; sprinkle remaining 2/3 cup peanut butter cups over surface, pressing in slightly. Cool completely in pan on wire rack. Cut into squares or triangles.
Makes 36 brownies.