Preheat & Prep. To start, preheat the oven to 350 degrees F. Line a metal 9x13 pan with parchment paper and set aside.
Combine Butter & Sugars. In a large bowl, combine cooled melted butter, granulated sugar, and brown sugar. Stir until evenly blended (it'll look like wet sand!).
Mix In Eggs & Vanilla. Scrape down the sides of the bowl and add eggs and vanilla extract. Stir until evenly combined. It'll be darker in color and thinner.
Combine Dry Ingredients. In a medium bowl, combine gluten-free flour, cornstarch, baking powder, cream of tartar, cinnamon, and salt. Whisk until evenly blended.
Add Dry Ingredients To Wet Ingredients. Mix the dry ingredients into the wet ingredients, and stir until just blended. The dough will be more like very thick batter at this point.
Prep For Baking. Transfer the cookie dough to the prepared pan and use an offset spatula to smooth dough into an even layer. (You can also gently tap the pan on the counter to level it out, if that's easier!) In a small bowl, combine sugar and cinnamon, then sprinkle over the cookie dough. It'll seem like a lot, but you want a good, even layer!
Bake Bars in the preheated oven 25-28 minutes, or until the top looks evenly set and a toothpick inserted in the center comes out clean with just a few moist crumbs.
Cool, Then Slice. Place the pan on a wire rack and let the bars cool completely before slicing. Cut into 12-24 slices and enjoy! Store leftover snickerdoodle bars in an airtight container at room temperature 2-3 days, or freeze up to 2 months.