Preheat the oven to 325F and grease a 12 cavity muffin pan very well (or line with parchment or silicone liners).
In a large bowl, whisk together the almond flour, coconut flour, baking powder, and salt.
In another large bowl, beat the butter with the sweetener until well combined. Beat in the egg, lemon zest, and vanilla extract, then beat in the almond flour mixture until the dough comes together.
Roll the dough into 12 even balls and place in the prepared muffin pan. Press evenly into the bottom and up the sides of the muffin cups.
Bake 20 to 25 minutes, or until the cups are puffed and the edges are just golden brown (they won't be completely firm to the touch yet but they shouldn't be mushy either!).
Remove from the oven and re-shape the cookie cavities with the end of a wooden spoon. Let cool in the pan for 15 minutes, then gently loose with a knife and transfer to a baking rack (if you baked them in parchment or silicone liners, they will be easy to remove).
Lemon Pastry Cream:
Bring whipping cream to a simmer in a medium saucepan over medium heat. In a medium bowl, whisk egg yolks with sweetener and salt.
Slowly whisk about half of the hot cream into the yolks to temper, then return the yolk/cream mixture back to the saucepan and cook until thickened, 4 to 5 minutes more, whisking constantly. Watch it carefully and do not let it curdle.
Remove from heat and whisk in butter, lemon juice, lemon zest, and lemon extract. Sprinkle the surface with xanthan gum and whisk vigorously to combine. Spoon into the cups and refrigerate until set, about 2 hours.
Garnish with fresh berries and lemon slices.
Serves 12. Each serving has 3.07g NET CARBS.Food energy: 266kcal Total fat: 24.68g Calories from fat: 222 Cholesterol: 93mg Carbohydrate: 5.56g Total dietary fiber: 2.49g Protein: 5.64g