This is from one of my favorite cookbooks: "From Amish and Mennonite Kitchens" by Phyllis Pellman Good and Rachel Thomas Pellman.

I made it today and put in raisins soaked in vanilla, and chopped walnuts. We'll just eat them as plain cookies instead of making the filling and making whoopie pies.

2 cups brown sugar
3/4 cup butter or shortening
2 eggs
1/2 teaspoon salt
1 teaspoon cinnamon
1 teaspoon baking powder
3 tablespoons boiling water
1 teaspoon soda
2 1/2 cups flour
2 cups oatmeal
(Raisins soaked in 1 teaspoon vanilla)
(1/3 cup chopped walnuts)

Cream sugar and shortening. Add the eggs, then add salt, cinnamon, and baking powder. Dissolve the soda in the boiling water and add it. Gradually stir in the flour and oatmeal, and the optional raisins and nuts.

Drop batter by heaping teaspoons onto greased cookie sheets and bake at 350 degrees for 8 to 10 minutes until brown.

Filling:
1 egg white, beaten
2 tablespoons milk
1 teaspoon vanilla
2 cups powdered sugar, divided
3/4 cup shortening

Mix the egg white, milk, and vanilla with 1 cup of the sugar. Then add the second cup of sugar and the shortening. Spread a dab of the filling on the flat side of a cooled cookie and top with another cooked to form a sandwich pie.