Preheat oven to 350 degrees. Line a baking sheet with parchment paper.
In a mixing bowl, add the butter and sugar. Mix on medium until light and fluffy.
Add in the egg whites and continue mixing on medium until incorporated.
Add in the sour cream and vanilla and continue mixing on medium until combined.
Add in the baking soda, baking powder, and flour. Mix until just incorporated.
Place a straight tip in a piping bag. Then run some gel paste food coloring on both sides of the bag using a knife. Add in the white whoopie pie batter. I would recommend adding all of it so make sure you have a decently large bag.
Pipe out nice round mounds of batter. Try to make the mounds the same size.
Place in the oven and bake for 7 minutes or until done.
In a mixing bowl, add in the butter and whip on medium speed.
Add in half of the powdered sugar and mix on low until the powdered sugar is combined with the butter. It will be thick.
Add in the milk and vanilla and mix on medium until incorporated.
Add in the remaining powdered sugar and turn the mixer to low. Let it go until the powdered sugar is incorporated and then turn it to high and let it whip for a minute.
Match up the cooled whoopie pie cookies so the top and bottom are about the same size.
Place a 1M tip in a piping bag and add the frosting to it.
Pipe a nice circle around the whoopie pie cookie and place the other cookie on top. Repeat to all cookies.