Prepare the pan: Preheat oven to 350°F (180°C ). Lightly grease an 8-inch (20 cm) springform pan, line the bottom with parchment paper, and grease the paper as well.
Melt chocolate and butter: In a heatproof bowl set over a pot of gently simmering water (double boiler), melt the chocolate and butter together, stirring until smooth. Remove from heat and let cool slightly.
Add egg yolks: Stir the egg yolks into the melted chocolate mixture until smooth and glossy.
Whip egg whites: In a separate clean bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the sugar and continue beating until stiff, shiny peaks form.
Combine mixtures: Whisk one-quarter of the whipped egg whites into the chocolate mixture to lighten it, then switch to a whisk and gently fold in the remaining egg whites in two additions until fully incorporated.
Bake: Pour the batter into the prepared pan, smooth the top, and bake for 25–30 minutes, or until the edges are set but the center is slightly soft, a toothpick should come out with a few moist crumbs.
Cool and chill: Let the cake cool completely in the pan, then refrigerate for at least 2–3 hours before serving. Dust with cocoa powder or powdered sugar if desired, and serve with whipped cream or ice cream.
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Notes
Chocolate: Use good-quality dark chocolate (70% cocoa or higher), since it’s the main flavor here, choose one you genuinely enjoy eating. Avoid chocolate chips or baking chocolate with additives.
Texture: Don’t overbake, the cake should remain slightly fudgy in the center. It will firm up as it cools.
Add-ins: You can fold in a splash of espresso, a teaspoon of vanilla extract, or a pinch of cinnamon or chili powder to enhance flavor. For a nutty twist, add chopped hazelnuts or almonds.
Serving: This cake is rich and dense, serve small slices! It pairs beautifully with whipped cream, vanilla ice cream, or fresh berries.
Storage: Store covered in the refrigerator for up to 4–5 days. Bring to room temperature before serving for the best texture. You can also freeze it (well wrapped) for up to 2 months.
Variation: For a mocha version, replace 1 tablespoon of butter with 1 tablespoon of espresso or strong coffee.