Prepare. Heat the oven to 175 °C (350 °F). Grease an 8-inch (20 cm) round cake pan and line the bottom with parchment paper.

Melt. Melt 150 g of the chocolate with the butter in a bowl over barely simmering water, stirring until smooth. Take it off the heat and let it cool for 5 minutes.

Whisk the eggs. Whisk the eggs, sugar and vanilla for 2–3 minutes, until pale and slightly thickened.

Combine. Pour the warm chocolate into the egg mixture in a thin stream, stirring constantly, until smooth.

Fold in the flour. Sift the flour, baking powder and salt over the bowl and fold them in with a spatula until no streaks remain.

Bake. Pour the batter into the pan and bake for 22–28 minutes, until a toothpick inserted in the center comes out with moist crumbs. Cool in the pan for 10 minutes, then turn out onto a rack and cool completely, about 45 minutes.

Make the ganache. Heat the cream until it just simmers and pour it over the finely chopped chocolate. Wait 1 minute, then stir slowly from the center until smooth and glossy. Stir in the butter, if using. Let it cool to 32–35 °C (90–95 °F), about 10 minutes.

Glaze. Set the cooled cake on a rack over a tray. Pour the ganache onto the center and nudge it to the edges with an offset spatula so it drips down the sides. Let it set at room temperature for 20–30 minutes before slicing.