Prepare brownie mix as directed on package in 13x9-inch baking pan. Cool on wire rack.
For the Chocolate Mousse, microwave chocolate in medium microwaveable bowl on HIGH 1 minute; stir. Microwave additional 30 seconds at a time, stirring until chocolate is smooth and melted. Add 1/4 cup of the heavy cream; stir until smooth. Stir in vanilla. Let stand 10 minutes to cool slightly. Beat remaining 2 cups heavy cream and granulated sugar in large bowl with electric mixer on medium speed until stiff peaks form. Gently stir in 1/2 of the chocolate mixture with spatula until well blended. Gently stir in remaining chocolate mixture.
For the Peppermint Cream, beat heavy cream, confectioners’ sugar and peppermint extract in large bowl with electric mixer on high speed until soft peaks form.
Spread 1/2 of the Peppermint Cream over brownie. Spread evenly with Chocolate Mousse. Top with remaining Peppermint Cream.