Prepare brownies according to package instructions for a 9x13 pan. Allow to cool completely.
To make the buttercream, in a large mixing bowl, combine butter, milk, vanilla, and 4 cups powdered sugar.
Mix until well-combined and fluffy. Continue adding powdered sugar or milk, as needed, so that frosting reaches a consistency that will spread easily, but will not run off the brownies.
Set aside 1/2 c. plain white buttercream for piping. Color the remaining buttercream with a few drops of green food coloring.
Spread the green frosting evenly on the brownies.
Fill the parchment bag, fitted with a #4 tip, with the white buttercream.
Following the tutorial instructions and photos (see blog post) to pipe the white lines and Hail Mary play onto the brownies. Enjoy!