Prepare the Pastry Cream: In a saucepan, heat the almond milk over medium heat until it begins to simmer.

In a bowl, whisk together egg yolks and sweetener until smooth. Stir in almond flour.

Gradually add the hot almond milk to the egg mixture, whisking constantly.

Return the mixture to the saucepan and cook over medium heat, whisking continuously, until it thickens (about 3-4 minutes).

Remove from heat and stir in butter and vanilla extract. Transfer to a bowl, cover with plastic wrap (touching the surface of the cream to prevent a skin from forming), and refrigerate until cooled and set.

Prepare the Donuts: Preheat your oven to 350°F (175°C) and grease a donut pan.

In a large bowl, combine almond flour, coconut flour, sweetener, baking powder, and salt.

In another bowl, whisk together melted butter, eggs, almond milk, and vanilla extract.

Gradually add the wet ingredients to the dry ingredients, mixing until just combined.

Spoon the batter into the prepared donut pan, filling each cavity about 3/4 full.

Bake for 12-15 minutes, or until a toothpick inserted into the center comes out clean. Allow the donuts to cool completely on a wire rack.

Once cooled, use a piping bag or a small spoon to fill each donut with the chilled pastry cream.

Dust with powdered erythritol if desired.