In a heavy-bottomed saucepan, gently heat the sugar over medium heat. Stir constantly to prevent burning. The sugar will clump initially but will eventually melt into a thick, amber-colored liquid.

Once fully melted, carefully add the butter. The mixture will bubble up, so stir continuously until the butter is completely melted and incorporated into the caramel.

Slowly drizzle in the heavy cream while stirring. Again, be cautious as the mixture will bubble. Continue stirring until you achieve a smooth, uniform caramel. Remove from heat and let it cool slightly.

Break the chocolate into small pieces and melt it gently. You can use a double boiler method or melt it in the microwave in 20-second bursts, stirring in between until completely smooth.

Choose your desired cake mold or cup for shaping your cake. Pour the slightly cooled caramel into the bottom, filling it about a third of the way up.

Let the caramel set slightly in the refrigerator for about 10 minutes. Once it has a bit of structure, carefully pour the melted chocolate over the caramel layer, filling up to the desired height of your cake.

Place the cake in the refrigerator to set completely. This could take anywhere from 1-2 hours, depending on the size and thickness of your cake.

Once fully set, carefully remove the cake from the mold. If needed, run a knife along the edge to help loosen it.

Serve as is or garnish with additional whipped cream, fresh berries, or a sprinkle of sea salt for an extra layer of flavor.