Position a rack to the middle of the oven. Preheat the oven to 375°F (165°C). Cut a piece of parchment paper to fit into the bottom of an 8-inch round cake pan. Grease the pan with 2 teaspoons of the butter, place the prepared parchment paper in the bottom of the pan, then grease it with 1 more teaspoon of the butter. Dust the inside of the pan with 1 teaspoon of the cocoa powder, tapping any excess around to cover.

In a medium mixing bowl, whisk together the granulated sweetener and 1/4 teaspoon salt. Set aside.

Fill a medium pot with 2 inches of water, then bring the water to a simmer. Put the chocolate chips and the remaining 8 tablespoons of butter in a stainless steel or glass bowl large enough to fit over the pot of water, then place the bowl over the water to create a double-boiler. Stir until everything melts and combines and the mixture is smooth and shiny. Transfer the chocolate mixture to the mixing bowl with sweetener and salt and stir to combine with a wooden spoon. Add the eggs one at a time, beating briefly to incorporate each one before you add the next. Mix until smooth. Stir in the vanilla and the cocoa powder; mix just until combined and thick, like brownie batter. Scrape the batter into the prepared pan and gently smooth the top.

Place the cake pan on a baking sheet pan and bake on the center rack for 25 minutes. Let cool in the pan for 5 minutes.

Using a butter knife, loosen the edges of the thin-crust-topped cake from the pan, then invert the cake onto a serving plate, so the top is now the bottom. Let cool completely, about 1 hour.

Make the chocolate glaze: Place the chopped baking chocolate in a heatproof bowl. Heat the heavy cream in a small skillet over medium heat, until it begins to produce fine bubbles around the edge and develops a skin on the surface. Pour the warm cream over the chocolate, stir briefly with a rubber spatula to combine, then let it rest for a few minutes. Stir again, starting slowly, then continuing more vigorously, until the chocolate is completely melted and the glaze is smooth and shiny. If any bits of chocolate remain, reheat briefly over a pot of simmering water, then stir until smooth. Pour the glaze over the cake, then spread it so it drips over the sides. Let the glaze set for 2 hours.