Place the butter into a large mixing bowl and whip using the whisk attachment on your hand mixer on medium-high speed for 2 minutes.
Slowly add the powdered sweetener, a tablespoon at a time, completely mix in before adding the next spoonful to prevent coating your kitchen in sweetener dust.
Once all the sweetener is mixed in, add the vanilla and the heavy cream, if using. The cream will soften the buttercream, and might not be needed depending on the temperature of where you are.
Continue whipping the buttercream until it is pale and fluffy. To check for the ideal piping consistency, take a spoonful of frosting and shake it back into the bowl. It should shake off the spoon in 2-3 firm shakes.
Using your frosting for icing cakes, cupcakes, or just eating by the spoonful.