Preheat the oven to 400 degrees fahrenheit. Prepare a sheet of parchment paper with a 9x11 inch pan.
In a mixing bowl, cream the brown sugar, salt and butter together for a few minutes. Add egg, vanilla and buttermilk to the butter mixture and mix together again.
Add the flour, cocoa powder, baking soda, and baking powder and then whisk together until combined.
Use a large ice cream scoop to place the finished batter onto the parchment paper.
Bake in the oven for 15-17 minutes until risen and looking fluffy.
While they are baking make the strawberry buttercream.
Cream the butter and salt in a mixing bowl for a few minutes until the butter has slightly lightened. Add the powdered sugar, vanilla extract and strawberry extract. Mix again with a hand mixer until combined.
Then add in the buttermilk and a drop of pink food dye if you want a pink frosting to show that it is a strawberry buttercream. Blend this together until a creamy frosting has formed.
Once the whoopie pies are out of the oven, let cool for 15 minutes.
Spread the strawberry buttercream onto each whoopie pie with a knife or use a piping bag. Place the other side of the pie on top of the frosting to create a sandwich.