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