Stencil heart shapes all over parchment paper. Then flip it over and place it on the baking sheet. This way the batter doesn’t go directly onto the pencil markings. Whisk the flour, cocoa, baking soda, and salt together in a medium bowl. Set this aside.
Add the red food coloring and mix until combined. Combine the flour mixture into the wet ingredients and then mix at low speed. Add in the vinegar and vanilla extract. Mix again until just combined.
Make the filling by beating the cream cheese with the butter until it becomes light and fluffy. Then add the powdered sugar in 1 cup at a time, beating after each addition.Pair up the cooled cookies that are similar in shape and size. Next, pipe the creamy filling onto the flat side of one of the cakes and then top it with the matching cake. Continue this assembly process until all the whoopie pies are finished.
I find that it works best to pipe the outline of the heart first and then fill it in. Afterward, I smooth the batter out carefully with a spatula.