Add parchment paper to a 9-inch springform pan and grease it with butter. Cover the bottom and sides on the outside with foil.
In the bowl of an electric mixer fitted with the wire attachment, beat eggs at medium speed for about 5 minutes. They should be light and fluffy.
Add cream, coffee, and sweetener to a small saucepan over medium heat. Simmer and stir until the sweetener is fully dissolved.
Add the chopped sugar-free chocolate and butter to a large bowl, pour the cream mixture over and stir until chocolate is fully melted and smooth. Stir in vanilla extract.
Using a rubber spatula, fold ⅓ of the egg foam into the chocolate mixture, stir gently, but quickly, to not cook the eggs, from the warm chocolate mixture. Stir in until only a few streaks of egg are visible and fold in half of remaining foam, then last of remaining foam, until mixture is fully combined.
Transfer batter into the prepared pan and smooth surface with a rubber spatula.
Place the cake pan into a roasting pan, transfer to the oven and pour enough boiling water to come about halfway up of the pan.
Bake for about 45 minutes, until the cake has risen slightly, edges are just beginning to set, an instant thermometer inserted in the center of the cake must read 140 degrees. The center of the cake should feel settled when you touch it; the edges should come off slightly from the sides of the pan.
Remove cake pan from the water bath and set on a wire rack, cool to room temperature. Remove from cake pan, and cool in the refrigerator for at least 3 hours or overnight before serving.
Combine raspberries, lemon juice and sweetener in a small saucepan set over low heat. Cook until berries release their juices and just start to break down about 4-5 minutes.
If you would prefer the sauce to be thicker, mix cornstarch with 2 tablespoons of water and add to the sauce. Cook until mixture comes to a boil. Let cool. Store, refrigerated, in an airtight container for up to 3 days.
Slice the cake and serve with whipped cream and raspberry sauce.