Preheat oven to 350°F and line two large baking sheets with parchment paper.
In a medium bowl, combine the gluten-free all-purpose flour, cream of tartar, baking soda, and salt. Whisk well to combine and set aside.
In a large mixing bowl, whisk the sugar and coconut oil together until you get a light and creamy mixture.
Add the beaten egg and vanilla extract and whisk to get a smooth liquid mixture.
Add the flour mixture to the wet ingredients and whisk well until a soft dough is formed. Let dough chill for 5 minutes in the freezer.
Once the dough is chilled, scoop out dough by teaspoon and roll into small balls (approx. 1 ½ inch balls) and place them on a plate. You should get about 18 to 20 balls in total. Let the balls chill in the freezer for 5 minutes.
As the cookie dough balls are chilling, mix the cane sugar and ground cinnamon together in a shallow bowl until combined.
Roll the balls in the cinnamon-sugar mixture until they are coated all over. Place the coated balls on a plate and let them chill in the fridge for 5 minutes.
Place the balls on the prepared baking sheets at least 3 inches apart (roughly 9 balls per sheet) and bake for 12 to 15 minutes until the edges are golden.
Let the snickerdoodles cool for at least 2 minutes on the baking sheets before removing them to cool completely on a wire rack.
*** CREAM OF TARTAR: Don't forget to add this ingredient as it enhances the flavour.