Prep the pan and oven. Heat the oven to 325°F (163°C). Grease an 8-inch round cake pan or springform pan. Line the bottom with a parchment circle, then grease the parchment too.

Melt chocolate and butter. In a heatproof bowl set over a saucepan of barely simmering water, melt the chocolate and butter together, stirring until smooth. Remove from heat and let it cool for 5–7 minutes so it’s warm, not hot.

Whisk the dry ingredients. In a small bowl, whisk cocoa powder, sweetener, salt, and espresso powder (if using) to break up any clumps.

Beat the eggs. In a medium bowl, whisk the eggs and vanilla until slightly foamy. You don’t need stiff peaks, just a bit of aeration for lift.

Combine mixtures. Stir the dry mixture into the melted chocolate until smooth. Slowly stream in the eggs, whisking constantly. The batter should look glossy and thick.

Pour and smooth. Scrape the batter into the prepared pan and smooth the top with a spatula.

Bake. Bake for 22–28 minutes, until the edges are set and the center looks just slightly soft with tiny cracks. A toothpick should come out with a few moist crumbs, not wet batter.

Cool and release. Let the cake cool in the pan for 10 minutes. Run a thin knife around the edge, then invert onto a plate and peel off the parchment. Flip again onto a serving plate so the top faces up.

Finish and serve. Dust lightly with keto powdered sweetener or cocoa. Serve plain, with berries, or with whipped cream. For the best texture, let it rest 30–60 minutes before slicing.