Preheat the oven to 375ºF/190°C.
Line the muffin pan with liners and set aside.
Whisk together almond flour, coconut flour, baking powder, salt, and cardamom in a large bowl.
In a medium bowl, whisk together egg, coconut oil, almond milk, stevia, and vanilla extract. Add to dry ingredients and mix until just combined.
Gently fold in the walnuts.
Divide the batter evenly into prepared muffin tins, filling each about ¾ of the way full. Sprinkle tops with additional chopped walnuts (optional).
Bake for 14-17 minutes, or until toothpick inserted into middle comes out clean.
Let cool for about 5 minutes, then remove muffins to a wire rack. Let cool completely or enjoy warm!