In medium saucepan, melt together mascarpone cheese and butter.
Whisk in egg yolks.
Continue to cook on low heat, stirring occasionally until custard thickens.
Remove from heat. Stir in liquid sweeteners and lemon flavoring.
Beat egg whites with cream of tartar until stiff peaks form.
Beat yolk mixture into egg whites.
Chill. If desired, beat in xanthan gum or glucomannan powder with an electric mixer to give more of a pudding consistency.
You can use an alternate sweetener to replace the stevia and monk fruit. In total, you'll want about 1/3 cup sugar equivalent or a bit more to taste.If adding xanthan gum or glucomannan powder, start with 1/4 teaspoon and add more as needed to reach desired thickness.