Oatmeal Whoopie Pies

1 . Stage

Preheat the oven to 350 degrees F (175 degrees C). Lightly grease 2 baking sheets.

2 . Stage

Make the cookies: Beat brown sugar and butter in a large bowl with an electric mixer until light and fluffy. Add eggs and mix well.

3 . Stage

Mix together flour, cinnamon, baking powder, baking soda, and salt in a separate bowl. Add flour mixture to wet ingredients in 2 batches, alternating with boiling water, mixing batter after each addition until well combined. Stir in oats.

4 . Stage

Drop spoonfuls of dough 2 inches apart onto the prepared baking sheets.

5 . Stage

Bake in the preheated oven for 10 to 12 minutes, switching racks halfway through. Remove from the oven and transfer to wire racks to cool.

6 . Stage

While the cookies are cooling, make the filling: Beat 1 cup confectioners' sugar, egg white, milk, and vanilla until combined. Beat in shortening and remaining 1 cup confectioners' sugar until light and creamy.

7 . Stage

Spread filling onto 1/2 of the cooled cookies, then sandwich with remaining cookies.