Mix almond flour, coconut flour, baking powder, and salt in a bowl.
In another bowl, combine eggs, erythritol, vanilla extract, melted butter, and milk.
Add the wet ingredients to the dry ingredients and gently mix until smooth.
Divide the batter into a donut pan.
Bake for 15 minutes in a preheated oven at 180°C (356°F).
Let cool, then sprinkle with powdered sugar or coconut flour as desired.