Preheat oven to 350˚F.
Grease a 9-inch springform pan with some butter and line the bottom with parchment paper. Set aside.
Pour water in a small saucepan; water should come up about 2 inches; set it over medium heat and bring to a simmer; decrease heat to low.
Put a metal bowl (or a heat-proof bowl) over the saucepan; to the top bowl, add the chocolate and butter; using a rubber spatula, stir until melted and smooth. Remove from heat and let cool slightly.
In the meantime, in a separate large mixing bowl, beat egg yolks with an electric mixer for 5 minutes, or until thickened and bright yellow.
Gradually beat in the melted chocolate mixture. Set aside.
In a separate large bowl add the egg whites and cream of tartar.
Clean the beaters and beat the egg whites on medium speed until foamy.
Gradually add the powdered sweetener or sugar, one tablespoon at a time. Continue to beat on high after each addition until the sugar is completely combined.
Add in the salt and vanilla, and continue beating on high until stiff and glossy peaks form.
Using a spatula, fold ¼ of the egg whites mixture into the chocolate mixture.
Then, fold in the remaining egg whites. Fold, do not mix, until incorporated, smooth, and chocolaty.
Transfer the mixture to the springform cake pan.
Bake for 30 to 32 minutes, or until a toothpick inserted in the center of the cake comes out with moist crumbs. Start checking for doneness around the 28-minute mark.
Remove the cake from the oven and let it cool on a wire rack.
Once cooled, loosen the sides of the cake by running a knife all around the edges of the pan.
Remove the rim from the pan.
Dust the cake with cocoa powder or confectioner’s sugar.
Add chocolate shavings and raspberries.