Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a large mixing bowl, beat the softened unsalted butter with brown sugar and granulated sugar until light and fluffy.

Add the room-temperature eggs one at a time, mixing well, then stir in the vanilla extract.

In a separate bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon.

Gradually add the dry mixture to the creamed mixture, mixing on low speed until just combined.

Fold in the old-fashioned oats until evenly distributed.

Drop rounded portions of dough onto the prepared baking sheets, spacing about 2 inches apart.

Bake for 10-12 minutes until edges are golden and centers are slightly underbaked.

Cool the cookies on the baking sheets for 5 minutes before transferring to a wire rack.

Prepare the filling by beating the butter until creamy, then gradually add powdered sugar and mix in the heavy cream and vanilla until smooth.

Spread the cream filling on one cookie and press another cookie on top.