1. In a large bowl, or stand mixer fitted with paddle attachment, cream butter and sugar until light and fluffy. 

2. Add in egg yolks, one at a time and beat on medium speed until combined.

3. Add in sour cream and vanilla and beat just until combined.

4. In a medium-sized mixing bowl, combine flour, baking soda, salt and cocoa powder. Mix well to combine.

5. Gradually add to butter mixture, mixing on low speed until well combined.

6. Scrape down the side of the bowl and ensure mixture is thoroughly combined. 

7. Using a heaping tablespoon size, roll into balls and place on parchment paper lined baking sheet. 

8. Bake at 350F for 8 minutes.  Using the back of a spoon, remove cookies and flatten slightly, then return to oven for two more minutes.

9. Remove from oven and let cool in pan for 2 minutes, then move to wire racks and cool for 15 minutes.

10. Meanwhile, melt the chocolate chips in the top of a double-boiler just until melted. If you do not have a double-boiler, you can melt directly in saucepan over low-medium heat being very careful not to scorch the chocolate. For both methods, stir constantly.

11. Dip the tops of half of the cookies into melted chocolate and immediately sprinkle with chocolate sprinkles. 

12. Place on plate in refrigerator to and chill for 15 minutes, until chocolate is set. 

13. Spoon 1 heaping tablespoon of marshmallow fluff onto bottom cookie. Using a butter knife, smooth the fluff, almost to the edges. Top with the chocolate-dipped cookie and press down lightly until filling is pushed to the edges.

Cookies may be made ahead and refrigerated in an airtight container for up to five days.

Makes 12 sandwich cookies