Place the sugar in a large bowl and when the butter has melted, pour it over the sugar. Let sit to cool slightly, 10 minutes.
Meanwhile, sift together the sweet rice, almond, oat, and tapioca flours with the cinnamon, cream of tartar, baking soda, and salt.
When the butter has cooled a bit, whisk in the egg and vanilla (if using) until the mixture is smooth and emulsified.
Stir in the flours and when they’re incorporated, beat the batter for 60 strokes. This will help the cookies bake up chewier.
Chill
Chill the dough until it’s firm enough to scoop, at least 30 minutes and up to 48 hours. The dough will become more smooth as it sits, allowing the starches to absorb moisture and bake up into thicker cookies.
Shape
When you’re ready to bake, position a rack in the upper third of the oven and preheat to 350ºF. Line two rimless cookie sheets with parchment paper.
Stir together the sugar and cinnamon in a small bowl.
If the dough has been chilled longer than an hour and is very firm, let it soften at room temperature for about 10 minutes so it's easier to scoop.
Use a cookie scoop to form the dough into balls (a #24 scoop, or about 2.5 tablespoons, will make 9 hefty cookies).
Roll each ball in the cinnamon sugar and place them on the prepared cookie sheets, spacing them at least 2 inches apart.
Bake
Bake the cookies at 350ºF, one sheet at a time, in the upper third of the oven until the edges are set but the centers are still underbaked, 8-10 minutes. The centers should be puffed and collapse slightly when touched.
Give the pan a firm rap on the counter for extra dense, chewy cookies.
Let the cookies cool on the pan for a few minutes to firm slightly, then remove them to a wire rack to cool completely. They will be delicate at first but will firm up as they cool.
Store covered at room temperature for up to 5 days. They're best same-day, with crisp edges and tender middles, but they will stay moist and soft for up to 5 days.
Notes
*Ingredient Substitutions:
In place of sugar, use ½ cup (75 g) maple sugar or coconut sugar
In place of butter, use vegan butter for dairy-free
In place of egg, melt 2 ounces cream cheese (dairy or plant-based) into the hot butter, then combine with the sugar.
The vanilla is optional; I slightly prefer the cleaner flavors of cookies made without it, but it's a traditional ingredient, so you do you!
In place of almond flour, sub by weight almond meal, hazelnut flour, tiger nut flour, or teff flour
In place of oat flour, sub by weight sorghum flour, millet flour, almond flour/meal, tiger nut flour, or teff flour
In place of sweet rice flour, sub by weight GF AP flour (such as Bob's Red Mill 1 to 1) or cassava flour
In place of tapioca flour, sub by weight arrowroot flour or cornstarch
In place of cinnamon, sub cardamom, or try my golden pumpkin pie spice
If you don't have cream of tartar, make these with 1 teaspoon baking powder in place of both the baking soda and cream of tartar