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

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.

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.

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

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.

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.

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