Cookies:

Preheat oven to 375 degrees and line large cookie sheet with parchment paper. In a bowl, mix together butter and both sugars with hand or stand mixer until light and creamy. Add egg, molasses, and Edmund Fitzgerald Porter, scraping the side of your bowl as necessary. In a separate bowl, combine flour, baking soda, and salt. Whisk until combined then slowly add to wet mixture. After wet and dry ingredients are completely combined, add in oats 1 cup at a time. Dough will be very thick, and it may be necessary to mix by hand. Make balls of dough about 2 tablespoons in size. Place at least 2 inches apart on pre-lined cookie sheet as they will expand during baking. Bake for 10 minutes at 375 degrees until edges are golden brown. Allow to cool on cookie sheet before moving to wire rack.

Filling:

In a bowl, mix butter until smooth with a stand or hand mixer. Add powdered sugar and mix until incorporated. Add whipping cream, vanilla, and marshmallow cream and mix on high for 4-5 minutes until filling is fluffy.

To Assemble:

Spread 2 tablespoons of filling in between 2 completely cooled cookies.