Prep the pan: Heat oven to 325°F (163°C). Grease an 8-inch round cake pan and line the bottom with parchment. Grease the parchment too.

Set up a water bath (optional but ideal): Wrap the outside of the pan with foil to prevent leaks. Place the pan in a larger roasting pan to create a bain-marie for gentle baking and a custard-smooth texture.

Melt chocolate and butter: In a heatproof bowl, combine chopped chocolate and butter. Melt over a double boiler or in the microwave in 20–30 second bursts, stirring until smooth.

Whisk in cocoa and sweetener: Sift cocoa into the warm chocolate mixture. Add sweetener and whisk until glossy and combined. If using espresso powder, add it now.

Add eggs and flavor: Whisk in vanilla and salt. Add eggs one at a time, whisking well after each until the batter is shiny and thick.

Pour and set the bath: Scrape batter into the prepared pan. If using the water bath, pour hot water into the roasting pan to reach halfway up the sides of the cake pan.

Bake: Bake 25–35 minutes. The edges should be set and the center slightly soft but not jiggly. A toothpick should come out with moist crumbs, not wet batter.

Cool completely: Remove the pan from the water bath. Let the cake cool in the pan for 15 minutes, then invert onto a plate, peel off the parchment, and flip again onto a serving platter.

Chill for best texture: Refrigerate for at least 2 hours (overnight is best) to set the crumb and deepen the flavor.

Finish and serve: Dust with powdered sweetener, slice with a hot, clean knife, and serve with a dollop of lightly sweetened whipped cream or a few berries.