In the bowl of a stand mixer, cream together the butter, sugar and salt on medium speed until fluffy and lighter in color.
In another bowl, sift together the cassava flour and baking powder. Be aware that cassava becomes airborne very easily, so sift it lightly.
Add the vanilla extract and egg to the butter-sugar mixture. Mix until homogenous, then add the cassava-baking powder mixture in small increments until it has all been added and a thick dough has formed.
Pour the dough onto plastic wrap, wrap it tightly and let chill for 1-2 hours.
Once the dough has chilled, cut the dough in half, reserving one half in the fridge. Using two pieces of parchment paper, begin rolling out the dough 1/4 inch thick. Cut out your sugar cookies and use a spatula to place the unbaked cookies on a parchment-lined baking sheet. The cookies will not spread, but try not to crowd the pan.
Once you’ve rolled out all your cookies, bake in the preheated oven for 10-12 minutes, or until the edges of each cookie turn golden brown. Let the cookies cool on the baking sheet (they will be brittle while still hot) and once cool, frost with either royal icing or frosting.