Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
In a small saucepan, bring the butter and water to a boil over medium heat. Once boiling, remove from heat and immediately add the almond flour, coconut flour, xanthan gum, and salt.
Stir vigorously with a wooden spoon until the mixture forms a ball and pulls away from the sides of the pan. Let the dough cool for 3 minutes so it does not cook the eggs. Add the eggs one at a time, beating vigorously after each addition until the dough is smooth and glossy. It should be thick but pipeable.
Spoon or pipe the dough into 8 equal mounds on the prepared baking sheet, spacing them about 3 inches apart. Wet your finger and smooth any peaks on top.
Bake for 10 minutes at 400°F, then reduce the oven temperature to 350°F (175°C) and bake for another 12 to 15 minutes until golden and hollow-sounding when tapped. Turn the oven off, crack the door open, and let the puffs dry inside for 10 minutes.
While the puffs cool, make the filling. Beat the heavy cream, powdered erythritol, and vanilla extract with an electric mixer until stiff peaks form.
Once the puffs are completely cool, slice each one in half horizontally and fill generously with the whipped cream. Serve immediately or refrigerate until ready to eat.
