Preheat the oven to 350°F. Butter an 9-inch tart dish or cake pan and line with aluminum foil.
Toast the hazelnuts in the oven or in a nonstick sauté pan on medium heat for 5 minutes. Cool for a few minutes, then place the nuts in a food processor and pulse until the nuts become powder, with a flour-like texture. (I used ground hazelnuts so I skipped this step)
Break the chocolate into pieces, and microwave it with the butter for 1 minute on high. Stir it — if the chocolate is not melted or close, repeat. Once everything is melted, stir in the cocoa powder.
Mix together the sugars, eggs, and vanilla in a large bowl until blended. Add the hazelnut powder and chocolate/butter mixture and fold them into the batter.
Pour the batter into a 9-inch round pan and bake for about 25 minutes (a toothpick inserted into the cake will come out with wet crumbs). Cool to room temperature on a rack.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.