Gather all the ingredients. Ensure that your mascarpone cheese or cream cheese and heavy cream are cold for the best results.
In a mixing bowl, add the mascarpone or cream cheese, powdered sweetener, peanut butter, vanilla extract, and a pinch of sea salt. Beat until smooth and well combined, about 2-3 minutes.
In a separate bowl, whip the cold heavy cream until soft peaks form. Be careful not to over-whip.
Gently fold the whipped cream into the peanut butter mixture using a spatula, starting with one-third of the whipped cream and then adding the rest.
Transfer the mousse into serving dishes or cups and refrigerate for at least 30 minutes to set.
Serve the mousse chilled, optionally topped with keto-friendly toppings like nuts or sugar-free chocolate.