Preheat the oven to 350 and line a cookie sheet with parchment paper.
Beat butter and sugar together for a couple of minutes, until light and fluffy.
Add the eggs, one at a time, beating after each addition.
Add vanilla, mix well.
Sift in the cocoa powder, flour, cinnamon, and baking soda. Mix on low speed until all combined.
Scoop out some cookie dough and roll into balls with your hands. (I use #50 or #40 cookie scoop.) Press down lightly and lay them out on the cookie sheet, a couple of inches apart.
Bake for 8-9 minutes and cool completely on a wire rack.
Chocolate Chip Cookie Dough:
Beat butter and sugar together for a couple of minutes, until light and fluffy. Add vanilla extract and milk, mix well.
Add flour and mix on low speed until all combined. Fold in mini chocolate chips.
Make sure the cookies are cooled before adding cookie dough and making them into sandwiches.
Dust cookie sandwiches with powdered sugar if you wish.