In a stand mixer, cream together the butter and granulated sugar until light and fluffy, about 3 minutes.

Add the eggs one at a time, mixing thoroughly after each addition, followed by the vanilla extract and melted chocolate.

In a separate bowl, whisk together the flour, cream of tartar, cocoa powder, baking soda, baking powder, and salt.

Gradually add the dry mixture into the wet mixture and mix on low speed until just combined.

Scoop the dough onto a lined baking sheet and refrigerate for at least 2 hours.

Preheat your oven to 350°F (175°C) and prepare additional baking sheets with parchment paper.

Mix together granulated sugar and ground cinnamon in a small bowl.

Roll each dough ball in the cinnamon-sugar mixture until fully coated.

Bake in the preheated oven for 12 minutes, or until edges are lightly cracked.

Let cool on baking sheet for 5 minutes, then coat in remaining cinnamon-sugar mixture.

Store in an airtight container or enjoy warm.