Prepare brownie mix according to package directions. Spoon batter into 13x9-inch baking pan, spreading evenly. Cover with 1 cup of M&M's.
In another mixing bowl, thoroughly beat the cream cheese with the sugar. Slowly add the heavy cream, eggs and vanilla extract. Blend mixture until smooth, scraping down the sides of the bowl several times.
Evenly spoon the cream cheese mixture over the brownie batter. Bake for 50-60 minutes, or until a toothpick inserted into the center of the pan comes out almost clean.
Remove and cool completely.
Optional: Just before serving, top with a layer of whipped cream.
Cut into 2-inch squares. Prior to serving, decorate with additional M&M's. Store in refrigerator.