Preheat your oven to 350 degrees. Line two cookie trays with parchment paper.
Cream together the brown sugar with the 6 tablespoons of softened butter. Add the egg and vanilla extract.
In a small bowl, whisk together the salt, cinnamon, baking powder, baking soda, and flour. Stir into the wet ingredients until fully incorporated.
Fold in the oats and hot water. Drop heaping tablespoons of dough onto your prepared sheets leaving 2 inches between each cookie. Bake for 10 minutes or until the edges are lightly browned. Cool completely.
For the filling: Whisk the butter until light and fluffy. Add the powdered sugar, cream, and vanilla. Beat until a creamy consistency is achieved. Spread 1 tablespoon of filling into the middle of two cookies