Preheat the oven to 300F and grease an 8 ounce ramekin very well.
In a medium heatproof bowl, combine the butter and unsweetened chocolate. Microwave on high in 30 second increments, stirring in between, until smooth (you can also melt it double boiler style).
Stir in the sweetener, vanilla and salt. Add the egg and whisk until fully incorporated. Pour into the prepared ramekin and bake 15 to 20 minutes. The sides will be set but the center will still be jiggly.
Remove and let cool completely, then refrigerate for at least 1 hour.
Meanwhile, combine the raspberries, sweetener and water in a small saucepan. Bring to a simmer and cook until the berries are soft enough to be mashed. Mash them up and let cool.
Loosen the edges of the torte with a sharp knife and flip out onto a plate. Drizzle with raspberry sauce and serve.