Preheat the oven to 400 degrees F and line several baking sheets with parchment paper. In a large bowl, whisk together the flour, cocoa powder, cream of tartar, baking soda, and salt. Set aside.
In the bowl of an electric mixer,cream the butter and 1 1/2 cups sugar together on high speed, until light and fluffy. (about 3–5 minutes) Then beat in the eggs and scrape the bowl with a spatula. Turn mixer on low and slowly add the flour mixture into the butter mixture, until just combined. Do not overmix the dough.
Mix the remaining 1/4 cup sugar and cinnamon together in a small bowl. Use a 1 1/2-tablespoon cookie scoop to measure out dough balls. Roll each ball in your hands to even them out, then roll in cinnamon sugar to thoroughly coat.
Place the balls on prepared baking sheets, 2 1/2 inches apart. Bake for 8–9 minutes, until just barely golden around the edges. Cool for several minutes on the baking sheets before moving.
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Notes
Gluten Free? You can absolutely use Gluten Free 1-to-1 Baking Mix in this recipe, instead of wheat flour.