In a bowl (or the greased ramekin/mug you want to use) mix together the peanut butter, butter, egg yolk and vanilla extract until well-combined.
Add the two sweeteners and mix well once again.
Add the almond flour, coconut flour, baking powder, xanthan gum and salt and mix again until you get a nice thick dough. (You can add 1-2 tsp heavy cream at this point for a gooier cake).
Press this dough evenly into the bottom of the ramekin or whichever greased mug you plan to use.
Microwave the cake for ~45s then top with additional chopped peanuts and peanut butter if desired and enjoy!