Melt chocolate in microwave for 30 second intervals until almost melted. Stir with a spoon until chocolate finishes melting. Add sweetener and mix into chocolate.
Add mascarpone and start mixing. You want to whip it fast with your spoon (similarly to beating eggs) so it's well combined, gets a mousse consistency and doesn’t split.
Spoon into individual ramekins or similar, or one bowl. Chill at least 20 minutes for ramekins and serve, one bowl will need a longer chill time.