Preheat the oven to 350 degrees F (175 degrees C). Grease 2 baking sheets.

Mix flour, baking soda, and salt together in a large bowl. Add melted butter, brown sugar, and white sugar; beat with an electric mixer until smooth. Add egg and vanilla; beat until combined. Mix in cocoa powder until incorporated. Stir in chocolate chips and cinnamon.

Drop hazelnut-sized balls of dough 2 inches apart onto the prepared baking sheets.

Bake in the preheated oven until edges are firm, 10 to 12 minutes. Cool on the baking sheet for 5 minutes before removing to a wire rack to cool completely.