Preheat oven to 360 degrees F / 180 degrees C. Line a 7 inch/ 18 cm cake tin with a circle of parchment paper. Grease well inside the pan.
In a small saucepan over medium heat combine butter, chocolate and xylitol. Whisk continuously, until all ingredients are melted and the mixture becomes smooth. Remove from heat and transfer to a large mixing bowl. Wait 10 minutes until the mixture cools down.
Add the eggs and vanilla. Whisk for about 1-2 minutes, until smooth. Gradually add cocoa powder and heavy cream, whisking gently with a rubber spatula.
Transfer the batter to the prepared pan. Bake for 20-30 minutes or until a toothpick inserted in the center of the cake comes almost clean and the edges of the cake are set.
Let cool for 10-15 minutes. Serve sprinkled with cocoa powder and garnished with fresh berries (both optional).
I make most of my keto desserts with xylitol but you can use erythritol, monk fruit sweetener or stevia, if you prefer. Beware that xylitol is as sweet as sugar so adjust the quantity of any other sweetener accordingly.
For non keto version you can use coconut sugar instead of xylitol, same quantity.
Ideally for this cake try to use the best quality chocolate and cocoa powder you can find.