Preheat the oven to 190℃ (170℃/375℉) and line two baking sheets with parchment paper.
Whisk together the flour, cinnamon, cream of tartar, baking soda, and salt. Set aside.
To a large mixing bowl, add the sugar and cooled melted butter. Whisk vigorously for 1-2 minutes, until the mixture lightens in colour and thickens slightly.
Add the egg, egg yolk, and vanilla, and whisk until just combined.
Add the dry ingredients in 3 batches, mixing between each addition. Switch to a rubber spatula when the dough becomes too thick to whisk. Do not over-mix!
In a small bowl or ramekin, whisk together the cinnamon sugar ingredients.
Scoop about 2 tbsp of dough, and roll between your palms to form a smooth ball. if you want to be precise, use a scale (mine are 35g each). Roll each dough ball twice in the cinnamon sugar, then place on a lined baking sheet, leaving at least 5 cm (2 inches) of space in between to allow for spreading.
Place the tray on the middle rack of the preheated oven. After 5 minutes, open the oven door and, using oven mitts, lift the tray a few inches and let it drop firmly back onto the rack (the cookies should deflate slightly). Close the door and bake for another 1–2 minutes until they puff up again. Repeat this lifting and dropping process once more. Bake for a total of 9–11 minutes until the edges are set but the centres remain slightly underbaked. Remove from the oven and give the tray one final drop on the countertop.
Let the snickerdoodles cool on the baking sheet for 5-10 minutes before transferring to a wire rack. Sprinkle with any leftover cinnamon sugar, if desired.