Special equipment: a 4-inch round cutter, a 2 1/2-quart freezer-safe bowl (about 9 inches by 4 inches) and a 3- to 4-cup freezer-safe bowl (about 5 inches by 2 inches)
For the brownies: Preheat the oven to 350 degrees F. Line a 12-by-17-inch baking sheet with parchment paper and spray with cooking spray.
Combine the granulated sugar, brown sugar, vanilla, salt and eggs in a large bowl and mix until well combined; set aside.
Melt the butter and chocolate in a double boiler, then whisk together until fully combined. Pour the chocolate mixture into the sugar mixture and mix until fully combined. Fold in the flour and cocoa.
Pour the batter into the prepared baking sheet and bake until a toothpick inserted in the center comes out clean, 18 to 20 minutes. Let cool completely.
For the bombe: Using a 4-inch ring cutter, cut a round from the baking sheet of brownies. Slice the remaining brownies into approximately 3-by-4-inch rectangles. You should have 10 rectangles. Keep 5 rectangles as they are and cut the other 5 in half to make triangles.
Line a 2 1/2-quart (about 9 inches across) freezer-safe bowl with plastic wrap. Place the brownie round in the bottom of the bowl and, alternating between the rectangles and triangles, fill the sides of the bowl. Press the brownie pieces into the bowl so that the interior of the bowl is lined with brownies. Break off any excess that extends above the edge of the bowl. Make sure to press the seams together so there are no cracks. Break the remaining brownie pieces into small crumbles and reserve.
Wrap a 5-inch freezer-safe bowl in plastic wrap from the bottom, combining the loose ends of the plastic wrap at the top.
Spread the softened ice cream inside the large bowl over the brownie layer. You should have about a 2-inch-thick layer of ice cream. Place the 5-inch bowl inside the ice cream layer, pushing it into the ice cream slightly to create a well. Cover the top with plastic wrap and freeze until the ice cream is firm, about 2 hours.
In a large chilled bowl, combine the heavy cream, granulated sugar and cocoa. Whip the cream with a whisk or electric hand mixer until stiff peaks form.
Remove the bombe from the freezer and remove the small bowl. Pour the hot fudge into the well, then fill with the whipped cream. Combine the reserved brownie crumbles and walnuts and sprinkle over the top of the bombe. Cover with plastic wrap and push down to press the brownie crumbles into the bombe. Freeze until the whipped cream is frozen, about 2 hours.
Remove the bombe from the freezer and place the bowl side into a large bowl of hot water for 1 minute. Remove the plastic wrap. Invert the bombe onto a large platter or plate. Remove the glass bowl and plastic wrap. Garnish with a sprinkle of cocoa powder. Slice and serve.