Preheat oven to 350°F and line cupcake pans with liners.

Combine flour, cocoa powder, baking soda, and salt in a medium bowl.

Cream together butter, sugar, oil, and vanilla in a large bowl until light and fluffy, about 3-4 minutes.

Add eggs one at a time, mixing well after each addition. Scrape down sides as needed.

Mix in half of the dry ingredients until combined.

Gradually add milk, mixing until incorporated. Batter may look curdled—that's okay.

Add remaining dry ingredients and mix until smooth. Do not overmix.

Fill cupcake liners about three-quarters full. Bake for 16-18 minutes, or until a toothpick comes out with a few crumbs.

Cool cupcakes for 2-3 minutes, then transfer to a wire rack to cool completely.

Core the cooled cupcakes to create space for filling.

Fill centers with marshmallow fluff using about 1.5 to 2 tablespoons per cupcake.

Prepare frosting: Beat butter until smooth, add melted chocolate and cocoa, then alternate adding powdered sugar and heavy cream until desired consistency.

Pipe frosting onto cupcakes and top with chocolate ganache and cookie crumbles. Serve and enjoy!