Preheat your oven to 325 degrees F and line a 12-cup muffin tin with paper liners. In a large bowl, whisk together the almond flour, 1/3 cup erythritol, and baking powder until no lumps remain.
Add the eggs, melted butter, 1/4 cup heavy cream, and vanilla extract to the dry ingredients. Stir until a smooth batter forms. Divide the batter evenly among the 12 liners, filling each about two-thirds full.
Bake for 18 to 22 minutes, until the tops spring back when lightly pressed and a toothpick comes out clean. Let the cupcakes cool completely in the tin before filling.
While the cupcakes cool, make the pastry cream. In a small saucepan over medium-low heat, warm 1 cup of heavy cream until it just begins to steam. Do not let it boil.
In a separate bowl, whisk together the egg yolks, 3 tablespoons erythritol, and xanthan gum. Slowly pour the warm cream into the yolk mixture in a thin stream, whisking constantly to temper the eggs.
Pour everything back into the saucepan and cook over low heat, stirring continuously, for 3 to 5 minutes until the mixture thickens enough to coat the back of a spoon. Remove from heat, stir in the vanilla extract, and transfer to a bowl. Press plastic wrap directly onto the surface and refrigerate for at least 1 hour.
Once the cupcakes are cool, use the handle of a small spoon to poke a deep hole into the center of each one. Spoon or pipe the chilled pastry cream into each cupcake until the hole is completely filled.
For the glaze, combine the sugar-free chocolate chips, 2 tablespoons heavy cream, and 1 tablespoon butter in a microwave-safe bowl. Microwave in 15-second intervals, stirring between each, until melted and glossy.
Spoon the warm chocolate glaze over the top of each filled cupcake, letting it drip naturally down the sides. Refrigerate for at least 15 minutes to set the glaze before serving.
