To Make The Whoopie Pies:

1. Preheat oven to 375 degrees. Line baking sheets with parchment paper or a Silpat.
2. In a bowl, sift together, flour, cocoa, baking soda, and salt. In another bowl, beat butter and sugar with a mixer on low until just combined. Increase speed to medium and beat for about 2 minutes. Add egg and vanilla and beat for two more minutes. Add half of the flour mixture and half of the milk and beat on low until incorporated. Repeat with remaining flour and milk and beat until combined.
3. Drop batter on baking sheet in tablespoon clumps, each about two inches apart. Bake for about 10 minutes each or until pies spring back when pressed gently. Remove from oven and cool for about five minutes before transferring them to a rack to cool completely. Once cooled, spoon or pipe the frosting onto the flat side of one cookie, and then top with another cookie to make a whoopie pie. Repeat with remaining frosting and cookies.

To Make The Biscoff Cream Cheese Frosting:

1. Using an electric mixer, cream together cream cheese and Biscoff spread on medium speed until combined. Add vanilla and powdered sugar, and mix on low speed until sugar is incorporated. Stop mixer, and use a spatula to scrape the sides of the bowl. Increase speed to medium and mix for 1 more minute until well combined.