1. Preheat oven to 350°. Cook first 2 ingredients in a large heavy-duty saucepan over low heat, stirring occasionally, 8 to 10 minutes or until melted and smooth. Remove from heat; cool completely (about 20 minutes).
2. Meanwhile, whisk together flour and next 4 ingredients in a large bowl until blended; stir in semisweet chocolate morsels.
3. Whisk together granulated sugar and next 3 ingredients until smooth. Whisk in bittersweet chocolate mixture. Whisk chocolate mixture into flour mixture just until combined.
4. Line bottom and sides of a 13- x 9-inch pan with parchment paper, allowing 2 to 3 inches to extend over sides; lightly grease parchment. Pour batter into pan.
5. Bake at 350° for 35 to 40 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Cool in pan.
6. Lift mixture from pan using parchment paper sides as handles; cut into 24 squares.
7. Split each brownie horizontally, and sandwich 1 Tbsp. chocolate frosting between top and bottom halves.