In a large mixing bowl, sift flour and cocoa powder a couple of times. Then, in a heatproof bowl or the bowl of a stand mixer, beat eggs with sugar with a hand whisk until frothy. Bring the bowl over a water bath or bain-marie and warm the mixture to 104°F (40°C), whisking constantly.

Remove the bowl from the heat and whisk eggs and sugar with a stand or an electric hand mixer for 10 minutes (longer if an electric mixer is used). The mixture will whiten and triple in volume and reach the ribbon stage.

Sift the flour and cocoa mixture over the bowl in a few additions and gently fold in using a rubber spatula. Pour the cake batter into the prepared baking pan and bake for 30-35 minutes. Remove the cake from the oven and let it cool in the pan for 10 minutes. Then, invert the pan and let the cake slip on a cooling rack. Let it cool down completely. 

To make the chocolate ganache, chop the dark chocolate bar (or use chocolate callets) and place it in a bowl. Alternatively, use a food processor to crush chocolate. Pour heavy cream into a small saucepan and bring it over medium heat, stirring with a hand whisk. Warm the cream to 176°F (80°C), then pour half over the chocolate.

With a wooden spoon, mix chocolate with cream gently from the center to the edges. Then add the remaining hot cream and stir until shiny and smooth. Leave to cool slightly, stirring occasionally, or film it in contact and let it set at room temperature. The ganache has to reach the spreadable stage.