Preheat your oven to 350°F (175°C) to ensure even baking.
In a medium bowl, whisk together the all-purpose flour, cocoa powder, baking soda, cream of tartar, ground cinnamon, and sea salt. Set aside.
In a large mixing bowl, use an electric mixer to cream the unsalted butter, packed brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next, then mix in the vanilla extract.
Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Avoid overmixing to keep cookies tender.
In a small bowl, combine 1/4 cup granulated sugar with 1 1/2 teaspoons ground cinnamon for rolling the cookies.
Using a cookie scoop or tablespoon, portion the dough into balls and roll each in the cinnamon sugar mixture until well coated.
Place the rolled cookie balls onto the cookie sheet about 2 inches apart and bake for 10-12 minutes until edges are set but centers are soft.
Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely for the perfect texture.