Place rack in center of oven; preheat oven to 375°F. Butter an 8-inch round cake pan, line bottom with parchment, and butter the paper.

Melt ½ cup fine-quality bittersweet chocolate, chopped and 8 Tbsp. unsalted butter in a double boiler or heatproof bowl over barely simmering water, stirring until smooth.

Remove from heat and whisk in ¾ cup sugar to combine. Add 3 large eggs and whisk until no streaks remain. Sift ½ cup unsweetened cocoa powder over chocolate mixture and whisk until just combined. Pour into prepared pan and bake until a thin crust forms, about 25 minutes. Cool in pan for 5 minutes, then invert onto a serving plate.

Sift additional cocoa powder on top of the cake and serve with Coconut Lime Sorbet if desired. Cake can be baked 1 week ahead; store in an airtight container.