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 until all combined.
Scoop out some cookie dough and roll into balls with your hands. Press down lightly and lay them out on the cookie sheet, a couple of inches apart.
Bake for 8-9 minutes and cool completely.
For cookie dough: Beat butter and sugar together for a couple of minutes, until light and fluffy. Add vanilla and milk, mix well. Sift in the flour and mix until all combined. Fold in the chocolate chips.
Make sure the cookies are cooled before adding the cookie dough and making them into sandwiches.