1. Butter six ring molds measuring 2 1/2 inches high and 2 3/4 inches in diameter. Place on baking sheet lined with parchment paper, and set aside.

2. Place the chocolate and butter in a medium heat-proof bowl set over a pan of simmering water, and heat until melted. Stir mixture to combine.

3. Combine the egg yolks and 6 tablespoons sugar in a large bowl, and whisk until mixture is pale yellow and thick, 3 to 5 minutes. Stir in the vanilla extract. Add the chocolate mixture to egg-yolk mixture, and stir to combine.

4. Place the egg whites in the bowl of an electric mixer fitted with the whisk attachment, and beat until frothy. Add the remaining 4 tablespoons sugar, and whisk until stiff peaks form. Fold egg-white mixture into chocolate mixture.

5. Spoon about 1/4 cup batter into each ring, and place a truffle in the center of each. Spoon the remaining batter over the truffles, and place in the freezer to set, at least 1 hour.

6. Preheat the oven to 350 degrees. Transfer the baking sheet with the filled ring molds to the oven, and bake cakes for 20 to 25 minutes, or until sides are set but center is still soft. Using a thin spatula, carefully transfer the cakes (still in the ring molds) to a serving plate. Lift the ring molds, and remove them. Serve the cakes warm with Earl Grey ice cream, and garnish with chocolate curls, if desired.