Preheat the oven to 350℉. Line baking sheets with parchment paper and set aside.
Cookies: In a large bowl, cream together the brown sugar and butter until light and fluffy. Add in the eggs one at a time, mixing after each addition.
Add in the salt, baking powder, baking soda, boiling water and flour, and mix until it is fully incorporated. Stir in the oats.
Using a medium cookie scoop, scoop the cookie dough onto the prepared baking sheets, making sure to keep them around 2 inches apart. Bake in the preheated oven for 11-12 minutes.
Remove cookies from the oven and allow to cool on baking sheet for 5 minutes before transferring to wire racks to cool completely.
Filling: In the bowl of a stand mixer, mix the butter until smooth, around 1 minute. Add in the powdered sugar, milk, vanilla extract and salt, and mix on medium speed for around 3 minutes or until light and fluffy.
Add the filling into a piping bag (or a large freezer bag with the corner cut off). Turn two cookies upside down and pipe the filling to cover the bottom. Place another cookie on top. Repeat with the remaining cookies.