Preheat oven to 350°F and line a 9x9-inch baking pan with parchment paper, leaving some overhang for easy lifting.
In a large mixing bowl, cream together softened butter and erythritol until light and fluffy, about 2 minutes.
Beat in eggs one at a time, then add vanilla extract and mix well.
In a separate bowl, whisk together almond flour, baking powder, salt, and cream of tartar.
Add the dry ingredients to the wet ingredients and mix until a thick dough forms.
Press the dough evenly into your prepared pan. Mix together the cinnamon-sweetener topping and sprinkle generously over the top.
Bake for 22-25 minutes until the edges are lightly golden and the center is set.
Let cool completely in the pan before lifting out and cutting into 16 squares. These taste even better the next day!