Preheat your oven to 350°F (175°C).
In a mixing bowl, combine almond flour, coconut flour, erythritol, and salt.
Stir in melted butter, egg, and vanilla extract until a dough forms.
Divide the dough into 8 equal portions and press each portion into the wells of a greased muffin tin, forming tartlet shells.
Bake for 12-15 minutes or until golden brown.
Allow the shells to cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
In a saucepan, whisk together lemon juice, lemon zest, erythritol, and eggs.
Cook over medium-low heat, stirring constantly until the mixture thickens, about 5-7 minutes.
Remove from heat and whisk in the butter pieces until fully incorporated.
Sprinkle xanthan gum over the curd and whisk vigorously to prevent clumps.
Allow the curd to cool slightly, then spoon into the cooled tartlet shells.
Refrigerate the tartlets for at least 1 hour to set the curd before serving.