Begin by browning the unsalted butter in a saucepan over medium heat until it turns a golden brown color and has a nutty aroma. Remove from heat and let it cool slightly.

In a large mixing bowl, combine the browned butter, light brown sugar, and granulated sugar. Mix until well combined.

Add the egg yolk, pumpkin purée, molasses, and vanilla extract to the sugar mixture. Stir until everything is well incorporated.

In another bowl, whisk together the all-purpose flour, baking soda, baking powder, ground cinnamon, ground nutmeg, ground cloves, and fine sea salt.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Fold in the quick oats until evenly distributed throughout the dough.

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

Scoop tablespoon-sized portions of the dough onto the prepared baking sheet, spacing them about 2 inches apart.

Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden. Remove from the oven and let them cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.

While the cookies are cooling, prepare the filling by beating the softened unsalted butter, marshmallow fluff, powdered sugar, vanilla extract, and a pinch of fine sea salt in a bowl until smooth and creamy.

Once the cookies are completely cool, spread a generous amount of the filling on the flat side of one cookie and top it with another cookie to create a sandwich.

Repeat this process until all cookies are filled.