Begin by making the cookies.
In a large bowl, cream the sugar and butter together using a hand mixer. Once light and fluffy, add the milk, cocoa, vanilla, and salt. Combine well.
Finally, mix the oats into the sugar mixture, coating each oat with chocolate. The mixture should be easy to work with.
Place heaping tablespoonfuls of dough onto a baking sheet lined with parchment paper. Flatten slightly into a cookie shape using the back of a spoon. Allow to sit for 1 hour to set up.
Meanwhile, make the Marshmallow Peppermint Frosting.
Place the marshmallows in a microwave-safe bowl and cook in 20-second bursts, stirring in between, until the marshmallows have fully melted. Allow to cool slightly.
Using a hand mixer, beat the lukewarm marshmallow along with the butter. Once combined, add the heavy cream, vanilla, peppermint extract if using, and the powdered sugar. Beat until light and fluffy.
Frost the top of each cookie. While the frosting is still tacky, sprinkle the tops with the crushed candy canes. Serve at room temperature or chilled.