Preheat your oven to 350 degrees. Line two baking sheets with parchment paper or silicone baking mats and set aside.

In a small bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.

In a large mixing bowl (or stand mixer with paddle attachment), beat the softened butter and brown sugar together on medium speed until light and fluffy, about 1 to 2 minutes.

Scrape down the sides of the bowl. Add the egg and vanilla extract. Mix until fully combined, about 30 seconds.

Add the dry ingredients to the butter mixture. Mix just until incorporated. Do not overmix.

Stir together the oats and hot water, then fold them gently into the dough until evenly distributed.

Drop heaping spoonfuls of dough (about 1 to 2 tablespoons each) onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 10 minutes, or until the edges are lightly golden. The centers should look slightly soft.

Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

Place the softened butter in a mixing bowl. Beat for 1 to 2 minutes until smooth and fluffy.

Scrape down the bowl. Add the powdered sugar, heavy cream, and vanilla extract. Beat on medium speed for 4 to 5 minutes, until light and fluffy.

Spread about 1 tablespoon of filling onto the flat side of one cooled cookie. Top with a second cookie and gently press together. Repeat with remaining cookies.

Serve immediately or store in an airtight container.