Preheat oven to 350ºF. Line two baking sheets with parchment paper.

To a medium sized bowl add the softened butter and beat until light and fluffy for about a minute. Add the brown sugar and granulated sugar. Cream together. Add the sourdough discard and egg yolk and mix for 1-2 minutes until fluffy and smooth.

To a small bowl, fluff together flour, cocoa powder, baking soda and salt. Add the flour mixture to the butter mixture and mix until the wet and dry ingredients are incorporated and form a cohesive dough.

Scoop the cookie dough into small balls, about 25 grams each. Roll each scoop into a ball and place on a baking sheet, spread apart, about 12-14 cookies per sheet. You will end up with 24-28 cookie dough balls.

Bake cookies in a preheated, 350º F oven for 10 minutes. After 10 minutes, the cookies should be puffed up and baked through. Let the cookies cool on a baking sheet for 5-10 minutes until they set up completely and are easy to remove from the baking sheet.

While the cookies cool whip together the marshmallow frosting. Mix butter, marshmallow fluff, powdered sugar, heavy cream, vanilla extract and salt together. Whip together for 2-3 minutes until light and fluffy.

Once cookies have cooled completely, scoop frosting into a piping bag or ziplock bag. Snip off the end of the bag and pipe frosting onto half of the cookies. Add the other cookie on top of the frosted cookie to make an Oreo sandwich. Press down lightly. Enjoy!