Preheat the oven: Preheat your oven to 350°F (175°C). Grease a mini muffin or tartlet pan with butter or non-stick spray.

Make the tart crust: In a medium bowl, mix together the almond flour, coconut flour, melted butter, powdered erythritol, vanilla extract, and salt until a dough forms. Divide the dough evenly among the muffin or tartlet cups and press it down firmly to form a tart shell. Prick the bottom of each tart with a fork to prevent puffing. Bake for 8-10 minutes, or until the edges are golden brown. Remove from the oven and let the tart shells cool completely.

Make the lemon curd: In a saucepan over low heat, whisk together the egg yolks, powdered erythritol, lemon juice, and lemon zest. Continue whisking as you slowly add the butter cubes, one at a time, until fully melted and incorporated. Cook for about 5-7 minutes, stirring constantly, until the mixture thickens enough to coat the back of a spoon. Remove from heat and stir in the vanilla extract. Let the lemon curd cool for about 10 minutes before filling the tart shells.

Assemble the tartlets: Spoon the lemon curd into the cooled tart shells, filling each one generously. Smooth the tops with a spatula or the back of a spoon. Refrigerate the tartlets for at least 1 hour to allow the lemon curd to set.

Serve: Garnish with a sprinkle of lemon zest or a dollop of whipped cream, if desired. Serve chilled and enjoy these refreshing keto-friendly treats!