To make the cookies, heat the oven to 350ºF. Line a baking sheet with parchment paper or a Silpat.
Place cold butter in a small bowl and microwave in 4 second bursts until it is slightly melted.
In a stand mixer fitted with the paddle attachment, beat butter on medium speed until light. Add the sugar and beat until light and fluffy, about one minute. Add the vanilla and egg and beat just until incorporated. Add flour, cocoa, soda, and salt, and pulse to prevent the dry ingredients from coming out the sides. Once the mixture starts to come together, mix on low speed just until fully incorporated. Fold in chocolate chips with a rubber spatula.
Roll golf ball sized mounds of dough between your hands and place at least two inches apart on the prepared baking sheet. Bake 8 minutes. Cookies will still be very soft but should no longer appear wet on top. Allow the cookies to cool 10 minutes on the baking sheet before very carefully removing to a cooling rack to cool completely. You want the cookies to be completely cool before frosting. You can place them carefully in the refrigerator to speed up this process.
To make the buttercream, beat butter in the bowl of a stand mixer fitted with the paddle attachment until light. Mix in vanilla and cinnamon. Add powdered sugar and heavy cream and mix until smooth. You can adjust the consistency of the frosting by adding more powdered sugar to stiffen or more heavy cream to loosen it up. If desired, add a few drops of natural red food coloring to achieve the pink color.
Spread a generous amount of frosting on the bottom of one cookie and gently press another against it to create a sandwich. Store cookies in an airtight container in the refrigerator for 3-5 days. Makes 8 sandwich cookies.