Prep the pan and oven. Heat the oven to 350°F (175°C). Grease an 8-inch (20 cm) round cake pan and line the bottom with parchment. Lightly butter the parchment, too.

Melt chocolate and butter. In a heatproof bowl set over a pot of barely simmering water, melt the chocolate and butter, stirring until smooth. Remove from heat and let it cool for 2–3 minutes.

Whisk in sugar, vanilla, and salt. Stir the sugar into the warm chocolate mixture until glossy. Add vanilla and salt. The mixture should be thick and shiny.

Add eggs one at a time. Whisk in the eggs, one by one, fully incorporating each before adding the next. The batter will look silky and slightly thick.

Fold in cocoa. Sift the cocoa over the bowl to prevent lumps. Gently whisk or fold until just combined. Do not overmix.

Bake. Pour into the prepared pan and smooth the top. Bake for 20–24 minutes, until the edges are set and the center is slightly soft with a thin, papery top.

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

Finish and serve. Dust with cocoa or powdered sugar. Serve slightly warm or at room temperature with berries or whipped cream.