Preheat oven to 325 degrees F.

Oil and lightly flour (coconut flour) a 9-inch springform cake round.

Cut a piece of wax paper or parchment to fit inside the bottom of the pan, then place the paper in the bottom of the pan (inside). Wrap the outside bottom of the pan with foil to protect it from the water bath.

In a double-boiler on gentle heat, melt the butter and chocolate together until smooth. Set aside to cool slightly.

In a clean mixing bowl (make sure there is no oil residue on the bowl or mixer attachments) beat the egg whites until they become cloudy and frothy; about 30 seconds. Continue beating while adding the erythritol and cream of tartar. Beat until stiff peaks form. (NOTE: Do not over beat. If the eggs curdle, throw them away and start over with new egg whites.)

Whisk the egg yolks, stevia and vanilla into the melted chocolate mixture in a large mixing bowl. Gently fold the egg whites into the chocolate (start by folding in about 1/3rd of the whites, then gently fold in the remaining whites), the mixture should end up fluffy and light.

Pour into the prepared pan.

Place the pan in a deep cooking sheet with about 1/2 to 1 inch of water in it.

Bake the cake for about 60-70 minutes or until a toothpick inserted comes out clean.

Remove cake from oven and allow to cool for about an hour.

Gently run a knife around the edge of the pan, and then carefully invert the cake onto a flat plate or other surface.

Remove the paper from the bottom (now the top) of the cake. Invert again onto the final plate for displaying the cake. (NOTE: The cake can be eaten right away but it may fall slightly when it is cut. For best results, it should be refrigerated for at least 6 hours before serving.)