The recipe I have calls for cocoa rather than chocolate chips. Also, my Mennonite friends always made these with a filling between two cookies. The recipe I have is from Lila Yoder and calls for: 1 egg white, beaten stiff 3C. powdered sugar 3/4 C. butter 1 tsp. vanilla (these were my sister's favorite)
I use marshmellow creme,butter and powdered sugar for a filling for my whoopie pies.....
For my filling, I use 1/2 C butter, 1/2 crisco, 1 cup sugar, 1 tsp vanilla and 1/2 cup milk- it isn't too sweet, so it showcases the cookies :)