How to Make It
Preheat the oven to 350°. Spray an 8-inch springform pan with nonstick cooking spray. In a medium bowl, whisk the flour with the baking powder, baking soda and salt. In a small bowl, whisk the cocoa powder with the water until a smooth paste forms. In the bowl of a stand mixer fitted with the paddle, beat the butter with the brown sugar at moderate speed for 3 minutes. Beat in the egg, egg yolk, buttermilk and vanilla. At low speed, beat in the dry ingredients and cocoa paste in 2 alternating batches.
Scrape the batter into the prepared pan and bake for 30 to 35 minutes, until a toothpick inserted in the center comes out with a few moist crumbs attached. Let the cake cool for 15 minutes, then remove the ring and let the cake cool completely on a rack. Leave the oven on.
Using a serrated knife, cut a ¼-inch-thick layer off the top of the cake. In a food processor, pulse the cake layer until fine crumbs form. Spread the crumbs on a small rimmed baking sheet and bake for 10 to 12 minutes, turning the pan halfway through, until the crumbs are crisp. Let the crunchies cool completely.
In a small saucepan, combine the cream with the brown sugar, cocoa powder and corn syrup and bring to a boil over moderate heat, whisking constantly. Reduce the heat to low and simmer for 1 minute. Remove from the heat and add the chocolate, butter and salt; whisk until smooth. Scrape the hot fudge into a bowl and let cool to room temperature.
Using scissors, trim a sheet of clear acetate to 6-by-24 inches. Wrap the acetate around the cake and secure it with tape. Secure the springform pan ring around the acetate-wrapped cake.
Using an offset spatula, spread half of the hot fudge evenly over the cake and top with half of the cake crunchies. Freeze until the fudge is firm, about 20 minutes.
Using the spatula, spread the chocolate chip ice cream over the fudge layer and top with the remaining cake crunchies; press down to help them adhere. Freeze the cake for 20 minutes, until firm.
Remove the cake from the freezer and spread the remaining fudge over the crunchies. Freeze for 20 minutes. Spread the strawberry ice cream over the fudge and freeze for another 20 minutes.
In the bowl of a stand mixer fitted with the whisk, beat 1 1/2 cups of the heavy cream with the granulated sugar until it holds soft peaks. Spread 2 cups of the whipped cream over the strawberry ice cream layer; refrigerate the rest of the whipped cream for decorating. Freeze the cake overnight.
Place the chocolate in a heatproof medium bowl. In a small saucepan, bring the remaining 1 cup of heavy cream to a boil. Pour the hot cream over the chocolate and let stand until melted, about 5 minutes. Stir until smooth, then tap the bowl on the counter to pop any air bubbles. Let the ganache cool down to 80°, about 30 minutes.
Carefully remove the springform ring and peel the acetate off the cake. Transfer the cake to a rack set over a rimmed baking sheet. Working quickly, pour the ganache onto the center of the cake in one fluid motion, allowing the excess to drip over the side. Freeze the cake until the ganache is firm, about 30 minutes.
Meanwhile, whisk the reserved whipped cream until stiff peaks form. Transfer to a pastry bag fitted with a star tip. Pipe whipped cream around the edge of the cake and garnish with maraschino cherries and rainbow sprinkles, if desired. Freeze the cake for 30 minutes before serving.