Line a 9-inch cake pan with plastic wrap, allowing enough overhang to go over the sides of the pan. Remove the ice cream from the freezer and allow it to soften slightly. Spread the ice cream evenly in the prepared pan. Cover the top with plastic wrap and return to the freezer and freeze completely, at least 4 hours.

In a saucepan, combine the root beer, cocoa powder and butter. Cook over medium heat until the butter is melted. Stir in the sugars and cook until dissolved. Remove from the heat and allow to cool. Stir in the root beer extract.

Preheat oven to 325°F. Grease 2 9-inch cake pans, line the bottoms with parchment paper or waxed paper, then grease the paper.

In a large bowl, whisk together the flour, baking soda and salt.

In another large bowl, slightly beat the eggs, then whisk in the root beer mixture. Gently fold in the flour mixture just until combined, being careful not to overbeat.

Pour the mixture evenly into the prepared pans. Bake until a tester inserted in the center comes out clean, about 30-32 minutes. Remove from the oven and allow to cool for 10 minutes, then remove from the pans and remove the parchment paper. Allow to cool completely.

Place the whipping cream in a large bowl and whip until slightly thickened. Gradually add the powdered sugar and continue to whip until thick with stiff peaks.

To assemble the cake, place one cake layer on a serving dish. Remove the ice cream layer from the freezer and remove the plastic wrap. Place the ice cream on top of the cake layer. Trim, if needed. Place the second cake layer on top of the ice cream. Frost the cake with the sweetened whipped cream. Garnish with grated chocolate, if desired.

Freeze until ready to serve.