Combine first 4 ingredients in a large bowl; beat with a wooden spoon 2 minutes. Stir in 1/3 cup semisweet chocolate morsels. Spread batter into a greased 10-inch springform pan. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pan on a wire rack.
Combine remaining 2/3 cup semisweet morsels, milk chocolate morsels, and whipping cream in top of a double boiler; bring water to a boil. Reduce heat to low; cook until chocolate melts, stirring often. Remove from heat; stir in butter until melted. Stir in pecans and vanilla; pour mixture into a medium bowl. Place bowl in larger bowl filled with ice. Cool mixture, stirring constantly, 6 minutes or until mixture is cool and slightly thickened.
Pour chocolate-pecan mixture evenly over cooled brownie layer; cover and chill at least 1 hour.
Run a knife around edge of pan, and release sides. Spread Chocolate Mousse evenly over chocolate-pecan layer. Chill 30 minutes before serving. Garnish, if desired. Store in refrigerator.