Center a rack in the oven and preheat the oven to 375ºF (190ºC). For a convection oven, reduce cooking temperature by 25ºF (15ºC). Butter an 8-inch (20 cm) baking pan. Dust the inside of the pan with flour and tap out the excess. I recommend using a 6-7 inch (15-18 cm) cake pan for a taller cake than mine.
Line the bottom with parchment paper, butter the paper, dust the paper with flour and tap out the excess.
Combine the flour and cocoa powder and sift, set aside.
Cut the chocolate bar into small pieces and set aside.
Set a heatproof bowl over a saucepan of simmering water (140ºF/60ºC) and add the chocolate. Stir occasionally until the chocolate is melted. Make sure not to introduce any oil or moisture to the bowl; otherwise, the chocolate will seize.
Add the coarsely chopped butter and let it melted completely. Transfer the bowl to the counter and let the mixture cool.
Separate the cold eggs, putting the whites in a mixer bowl or other large bowl and the yolks in a small bowl.
While the chocolate is cooling, place the egg whites in a clean bowl with salt. Working with the whisk attachment of the mixer, beat the egg whites until they are foamy and start adding half of the sugar (90 g) in 2-3 separate additions.
Continue to beat on high speed until stiff peaks form and sugar is dissolved (about 5 min total). To test for stiff peaks, the peaks should stand straight up when you lift up the beaters. The whites should not slide around. Transfer the meringue into a bowl and wash the mixing bowl for next step.
In the clean mixing bowl, beat the egg yolks and the rest of the sugar (90g) until creamy and stir in the heavy whipping cream.
With a rubber spatula stir the chocolate mixture into the egg mixture.
Then add the flour and cocoa powder into the mixture.
Using the spatula, stir about ⅓ of the meringue into the batter, then gently fold in the rest until the color is uniform.
Pour the batter into the pan and give the pan several sharp taps on the counter to bring up any air bubbles that may be trapped in the batter.
Bake the cake at 375ºF (190ºC) for 10 minutes. Then reduce the temperature to 340ºF (170ºC) and bake for 30-35 minutes until a toothpick pulls out moist crumbs when inserted near the center of the cake. Transfer the pan to a cooling rack and let the cake cool completely. As the cake cools, it may deflate a bit.
Run a knife along the edges of the cake and carefully turn the cake over onto a plate or cake stand. Decorate the cake with raspberry and dust the powdered sugar before serving.
You can keep the leftovers in an airtight container and store in the refrigerator for up to 3 days or in the freezer for a month.