Preheat the oven: Preheat your oven to 350°F (175°C). Grease a tart pan or pie dish with butter.

Prepare the crust: In a mixing bowl, combine the almond flour, coconut flour, melted butter, powdered erythritol, vanilla extract, and a pinch of salt. Mix until it forms a dough-like consistency. Press the dough into the bottom and up the sides of the prepared tart pan. Bake for 10-12 minutes, or until the crust is lightly golden. Remove from the oven and set aside.

Prepare the filling: In a skillet, melt the butter over medium heat. Add the sliced zucchini or chayote squash, powdered erythritol, cinnamon, nutmeg, lemon juice, and vanilla extract. Cook for about 10-15 minutes, stirring occasionally, until the zucchini becomes soft and absorbs the flavors. If you want a thicker filling, sprinkle a pinch of xanthan gum while stirring.

Assemble the tart: Spoon the zucchini filling into the pre-baked crust, spreading it evenly.

Make the topping: In a small bowl, combine the almond flour, softened butter, powdered erythritol, and cinnamon. Mix until crumbly, then sprinkle the topping over the filling.

Bake: Bake the tart in the preheated oven for 20-25 minutes, or until the topping is golden brown and the filling is bubbly.

Cool and serve: Allow the tart to cool for 10-15 minutes before slicing. Serve warm with a dollop of whipped cream or a scoop of keto-friendly vanilla ice cream.