In a mixing bowl, cream together the mashed avocado and truvia until smooth and well combined.
Beat in the egg and vanilla extract until fully incorporated. Then mix in the sour cream until the mixture is smooth.
In a separate bowl, whisk together the cornstarch, baking soda, baking powder, cream of tartar, salt, and flour.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Wrap the dough in plastic wrap and let it chill in the fridge for at least an hour.
On a lightly floured surface, roll out the dough to about 1/4 inch thickness and use cookie cutters so you can be able to cut your desired shapes.
Place the cookies on the prepared baking sheet and bake them in a 350°F (175°C) oven for 10 minutes or until the edges are lightly golden
Let the cookies cool on the baking sheet for 5-10 minutes before transferring them to a wire rack to cool completely.