Spray insides of 2 large microwave-safe mugs with cooking spray. Place 1 tablespoon peanut butter in bottom of each mug. Whisk together Reddi-wip, Egg Beaters and brownie mix in medium bowl. Place half of batter in each mug.
Microwave each mug individually on HIGH 1 minute to 1 minute 15 seconds or until set. Invert each cake onto a plate. Serve immediately with additional Reddi-wip, if desired.