Prep the pan: Heat the oven to 325°F (165°C). Butter an 8-inch round cake pan (preferably springform). Line the bottom with parchment and butter the parchment. Dust with cocoa and tap out the excess.

Melt chocolate and butter: In a heatproof bowl, combine butter and sugar-free chocolate. Set over a pot of gently simmering water (don’t let the bowl touch the water). Stir until smooth. Remove from heat and cool 5 minutes.

Whisk in dry flavor boosters: Whisk in cocoa powder, espresso powder, and salt until no lumps remain. The mixture will be thick and glossy.

Add sweetener and vanilla: Whisk in granulated keto sweetener (and powdered sweetener if using) plus vanilla. Taste a tiny bit and adjust sweetness if needed. Keep in mind sweetness softens slightly after baking.

Incorporate the eggs: Add eggs one at a time, whisking well after each until fully blended. The batter should be smooth and pourable, like thick brownie batter.

Fill the pan: Pour the batter into the prepared pan and smooth the top with a spatula. Tap the pan firmly on the counter to release air bubbles.

Bake gently: Bake for 22–28 minutes. The edges should be set, and the center should look slightly soft with a thin, shiny crust. A toothpick inserted 1 inch from the edge should come out with moist crumbs, not wet batter.

Cool in the pan: Set the pan on a rack and cool for 20–30 minutes. Run a thin knife around the edge, then release the springform or invert onto a plate and peel off the parchment.

Chill for best texture: For a clean slice and fudgy bite, chill the cake for at least 1–2 hours. Bring to room temp for 15–20 minutes before serving for maximum silkiness.

Finish and serve: Dust with cocoa, add whipped cream, or top with berries. Slice with a warm, thin knife for neat edges.