Preheat oven to 350 degrees F. Prepare Chocolate Ganache and Peanut Butter Paste. For brownies: In a medium saucepan, combine the 1/2 cup butter and the unsweetened chocolate. Heat and stir over low heat until melted. Remove from heat; cool.
Meanwhile, line a 15x10x1-inch baking pan with foil, extending the foil up over the edges of the pan. Grease and flour the foil; set aside. Stir sugar into cooled chocolate mixture in saucepan. Add the eggs, one at a time, beating with a wooden spoon after each addition just until combined. Stir in the vanilla.
In a small bowl, stir together flour and baking soda. Add flour mixture to chocolate mixture; stir just until combined. Stir in the finely chopped peanuts. Spread the batter in the prepared pan (the batter will be a thin layer in pan).
Bake for 15 minutes. Cool in pan on a wire rack. Remove brownies from pan by lifting foil. Using a 2-inch round cutter, cut 12 rounds from brownies (save remaining brownies for another use).
Preheat oven to 350 degrees F. Line a large baking sheet with parchment paper; set aside. Unfold phyllo dough; remove one sheet of the phyllo dough and place it on a clean work surface. (As you work, cover the remaining phyllo dough with plastic wrap to prevent it from drying out.) Brush sheet lightly with melted butter. Top with another sheet of the phyllo dough; brush with melted butter. Repeat with two more sheets of the phyllo dough and some more of the melted butter.
Using a sharp knife or pastry cutter, cut two 5-inch squares from the phyllo stack. Discard scraps. Place one brownie in the center of each square; top brownie with a rounded 1 teaspoon of the Peanut Butter Paste. Top with another brownie, pressing down slightly. Spoon 1 tablespoon of the Chocolate Ganache on top of the stacked brownies. Sprinkle with coarsely chopped peanuts. Carefully draw corners of phyllo up to barely meet in the center, press corners lightly into Chocolate Ganache. Brush phyllo with some more of the melted butter. Place packets on the parchment-lined baking sheet.
Repeat steps 5 and 6 with remaining phyllo, melted butter, brownies, Peanut Butter Paste, and ganache to make four more packets.
Bake for 12 to 15 minutes or until golden brown. Using a wide metal spatula, carefully transfer packets to a wire rack. Serve warm or cool to room temperature. Makes 6 servings.
Chop the semisweet chocolate; set aside. In a small saucepan, combine the whipping cream and vanilla; bring to a boil over medium-high heat. Remove from heat. Add the chopped semisweet chocolate all at once. Let stand without stirring for 5 minutes. Whisk until melted and uniformly dark. Cover and chill in the refrigerator about 2 hours or until ganache is firm and fudgelike. If ganache gets too firm, let stand at room temperature until slightly softened.
Peanut Butter Paste
In a small bowl, stir together peanut butter and powdered sugar until well mixed.