In a medium-sized bowl, whisk together flour, cornstarch, baking powder, baking soda, cinnamon, nutmeg, and salt. Set aside.
In a large bowl, whisk together sweet potatoes, butter, and granulated sugar. Add egg and vanilla extract.
Beat in flour mixture until well combined. Cover the bowl with foil or parchment paper and refrigerate for one hour or overnight.
Preheat oven to 350 degrees. Line 2 large baking sheets with parchment paper.
Scoop dough out with a small cookie scoop, then flatten with the bottom of a measuring cup (you may need to spray oil on it to keep it from sticking) and smash flat (around 1/2 inch thick.)
Bake the cakies for 10 to 12 minutes or until tops are set. Transfer to wire racks to cool.
TAHINI GLAZE OR TAHINI BUTTERCREAM FROSTING
*NOTE If you want to make tahini buttercream frosting instead of glaze, use softened butter instead of melted... everything else is the same.
Mix melted butter and tahini, then slowly sift in powdered sugar, salt, and cardamom a spoonful at a time, continuing until glaze is thoroughly mixed.
Put the tahini glaze in a piping bag and drizzle over cookies.Or if using buttercream, use a piping bag or spreading tool.(let cool completely before storing)