Preheat oven to 190°C.

Grease an 8-inch round cake pan and line the bottom with parchment paper.

Melt the chocolate and butter:

In a medium saucepan over low heat (or in a heatproof bowl over simmering water), melt the butter and chocolate together, stirring until smooth. Remove from heat and let cool slightly.

Mix in sugar, salt, and vanilla:

Stir in sugar, salt, and vanilla extract into the melted chocolate mixture.

Beat in the eggs one at a time, mixing well after each addition.

Add the cocoa powder and mix until just combined. The batter will be thick. Pour into the prepared pan.

Bake for 25 to 30 minutes, or until the top has formed a thin crust and a toothpick inserted in the center comes out with moist crumbs.

Let the cake cool in the pan for 10 minutes, then turn out onto a plate. Peel off parchment and let cool completely.