Watermelon Ice Cream Cake

1 (9-inch) white cake round (see related content)
2 1/2 cups raspberry sorbet
2 ounces semisweet chocolate, chopped fine
12 tablespoons unsalted butter, softened
3 tablespoons lime juice (2 limes)
1/2 teaspoon vanilla extract
1/8 teaspoon salt
2 cups (8 ounces) confectioners' sugar
Green food coloring

FOR THE CAKE: Line 6-cup-capacity mixing bowl with plastic wrap. Using serrated knife, cut off domed top of cake so that remaining cake measures 3/4 inch thick (reserve top for another use). Cut remaining cake into 8 wedges and line bowl with wedges, arranging as closely as possible to achieve snug fit. Scoop raspberry sorbet into separate bowl and mash with wooden spoon until softened; gently fold in chopped chocolate. Fill cake-lined bowl with sorbet mixture and smooth top. Cover surface with plastic wrap and freeze until very firm, about 6 hours.

FOR THE FROSTING: Using stand mixer fitted with whisk, whip butter, lime juice, vanilla, and salt on medium-low speed until combined. Slowly add sugar and continue to whip until smooth, about 2 minutes. Increase speed to medium-high and whip until light and fluffy, about 5 minutes. Add 6 drops food coloring and mix until combined. Transfer 1/2 cup frosting to small bowl, add 6 more drops food coloring, and whisk until combined.

TO FINISH: Remove plastic from cake and turn out onto chilled plate or cake pedestal; remove remaining plastic wrap. Cover cake with even layer of light green frosting. Spread 8 vertical stripes of dark green frosting evenly around cake. Freeze until frosting is firm, at least 30 minutes. Serve.

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