Cookies:

1. Preheat oven to 375 degrees. Line 2 cookie sheets with parchment paper or silpat liners.
2. In a bowl of a stand mixer beat together the butter, shortening, and granulated sugar until the mixture is light and fluffy, about 2 minutes. Add the eggs and mix until well incorporated.
3. Add the flour, cream of tarter, baking soda and salt. Mix until just combined.
4. In a small bowl combine the cinnamon and granulated sugar.
5. Roll the dough into small balls and roll in the cinnamon and sugar to coat. Place the balls of dough about 2 inches apart on the prepared cookie sheets. Bake 8 minutes. 
6. Remove the cookies immediately to a wire rack to cool completely.
7. Once the cookies have cooled completely, flip over half of the cookies and pipe a generous amount of the marshmallow buttercream on the flat side. Place another cookie on top and gently press together.
8. Store in an airtight container for up to three or four days (if they last that long).

Marshmallow Buttercream:

1. In the bowl of a stand mixer or large bowl using a hand mixer, beat butter until smooth. Add marshmallow creme, whipping cream, vanilla and pinch of salt. Beat until smooth and creamy.
2. Add powdered sugar and mix on low until powdered sugar is incorporated, scraping down the sides of the bowl as needed. Turn mixer up and beat until smooth.