Preheat your oven to 350°F and prepare two baking sheets by lining them with parchment paper.

In a medium bowl, mix the softened butter until light and fluffy, about one minute.

Add both the brown and granulated sugar, continuing to cream the mixture together.

Mix in the sourdough discard and egg yolk, stirring until the mixture is fluffy and smooth.

In a separate small bowl, combine all-purpose flour, cocoa powder, baking soda, and salt.

Add the dry ingredients to the butter mixture and combine until a cohesive dough forms.

Scoop small portions of the dough, about 25 grams each, and place them on the lined baking sheets.

Bake for approximately 10 minutes, watching for them to puff up and look baked through.

Let them cool on the baking sheet for 5 to 10 minutes before moving to a wire rack to cool completely.

While the cookies cool, prepare the frosting by whipping together softened butter, marshmallow fluff, powdered sugar, heavy cream, vanilla extract, and salt in a bowl.

Beat the mixture until it becomes light and fluffy, about 2 to 3 minutes.

Once the cookies have cooled, take half of them and pipe frosting onto the flat side.

Sandwich these with the remaining cookies, pressing gently to combine.