Preheat the oven to 375° F. Line a cookie sheet with parchment paper or a silicone baking mat.
In the bowl of a stand mixer or with an electric hand mixer combine the partially melted butter and brown sugar until combined.
Add the egg and vanilla, mixing well.
Add the flour, cocoa powder, cornstarch, soda and salt to the bowl. Slowly start mixing. As the dry ingredients are mixing in, slowly add in the buttermilk.
Mix until a thick batter forms.
Add the red food coloring, mixing and adding a little more at a time until you have reached your desired color.
Add the batter to a piping bag. Pipe round circles equal in size onto the prepared baking sheet, leaving room for the cookies to bake and spread, slightly. Alternatively you can scoop the batter onto the pan.
Bake at 375° F for 8-10 minutes or until the cookies have spread and no longer look wet and underbaked in the center of the cookie.
Allow the cookies to cool on the pan.
While the cookies are cooling, make the filling.
To make the filling, slowly cream the cream cheese and butter until smooth.
Add powdered sugar one cup at a time, mixing slowly until the powdered sugar is completely incorporated.
Repeat the last step until all 4 cups of powdered sugar has been added.
Add in the vanilla extract.
Add the filling to a piping bag. Pipe the filling onto the bottom of a cooled red velvet cookie and top it with another cookie to create the whoopie pie. Repeat with remaining cookies.
Store in an air tight container for up to 3 days at room temperature or up to 1 week in the fridge.