Preheat the oven to 200ºC (390ºF). Once the cookies go in, reduce the temperature to 180ºC (355ºF).

In a medium bowl, whisk the egg with the milk, melted butter, vanilla extract, and erythritol until smooth.

In another bowl, combine the almond flour, coconut flour, and oat flour, making sure there are no lumps.

Gradually add the dry ingredients to the wet mixture, stirring until a dough forms.

Knead the dough gently with clean hands. Shape it into a cylinder, wrap it in plastic wrap, and refrigerate for at least 1 hour (or overnight).

Once chilled, cut slices of the dough, roll them into small balls, and flatten them slightly.

Place the cookies on a parchment-lined baking sheet and bake for 10 minutes. Remove, flip them over, and bake for another 5 minutes, keeping a close eye to avoid burning.

Let the cookies cool completely on a wire rack before storing them in an airtight container

These cookies actually taste even better the next day, as the flavors develop and the texture improves with time.