Preheat the oven to 350° F.
Grease a baking sheet with a little butter.
Cream together the peanut butter, butter, and erythritol in a large mixing bowl.
Mix in the egg.
Add the almond flour, coconut flour, salt, and baking soda and mix well.
Drop by heaping tablespoonfuls onto the baking sheet.
Use a fork to create a crosshatch pattern on the top of each cookie, flattening out the cookies. The dough is a little sticky, so run the fork under water before doing this.
Bake for 15-20 minutes.
Cool a bit and enjoy warm or room temperature.