In a small mixing bowl, whisk together the flour, cocoa powder, baking soda, salt, cinnamon, and cayenne (if using). Note - I recommend sifting the ingredients into the bowl, then whisking. This helps remove any extra lumps.
In a medium mixing bowl, add the butter, brown sugar, and white sugar. Use a hand held mixer and beat on medium speed until combined and creamy. Add the egg yolk and vanilla extract and beat until the egg yolk is thoroughly combined.
Add the dry ingredients to the wet ingredients and beat on low speed until all of the dry ingredients are combined and no streaks remain.
Use a cookie scoop (or spoon) and scoop 8 equal dough balls. Roll the dough balls through the cinnamon sugar a couple of times, and place onto the prepared baking sheet, at least two inches apart. Bake for 9-11 minutes, until the edges are crisp, but the centers are still soft.
Cool on the baking sheet set on a wire cooling rack for 5 minutes. Then transfer the cookies to the wire cooling rack to cool completely.