For the cake, grease a 9-inch springform pan and preheat oven to 325F.
In a medium bowl, combine hazelnut meal, unflavored whey protein powder, baking powder and salt.
In a large bowl, beat together the butter and sweetener until well combined. Beat in hazelnut oil, egg, hazelnut extract and stevia extract.
Beat in hazelnut meal mixture, then beat in almond milk until batter is well combined.
Spread batter in prepared pan and bake 25 to 30 minutes or until top is set, lightly browned and a tester inserted in the center comes out clean. Let cool completely.
For the mousse, in a small saucepan over low heat, melt butter and chocolate together, stirring until smooth. Remove from heat and let cool to lukewarm.
In a small bowl, whisk egg yolks together until smooth. When chocolate is lukewarm, whisk in egg yolks. Mixture will become very thick and chocolate will seize. Add 2 tablespoons of hot water and whisk vigorously. Continue to add 1 tablespoon water at a time until chocolate mixture becomes smooth (will still be quite thick). Set aside.
In a large bowl, beat egg whites with powdered sweetener, the salt and the cream of tartar until they hold stiff peaks but are not dry.
In another bowl, beat cream with hazelnut extract until it holds soft peaks.
Stir a couple of large spoonfuls of the egg whites into the chocolate mixture to lighten it. Then carefully fold the chocolate mixture back into the egg whites. Fold in the cream thoroughly but gently.
Spread mousse over cooked cake in springform pan. Let set in refrigerator at least 2 to 3 hours.
To remove, run a sharp knife around the inside of the pan sides and gently loosen. Remove sides and serve.
Serves 12. Each serving has 6 g of carbs and 2 g of fiber. Total NET CARBS = 4 g.249 Calories; 23g Fat (80.9% calories from fat); 6g Protein; 6g Carbohydrate; 2g Dietary Fiber; 98mg Cholesterol; 179mg Sodium