Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch cast iron skillet with vegan butter.

Sift flour, cocoa powder, baking soda, and salt into a medium bowl.

Combine white sugar, brown sugar, and softened vegan butter in a large bowl. Add dry ingredients and mix until thoroughly combined. Stir in almond milk. Fold in 1/2 of the chocolate chunks.

Transfer 2/3 of the dough to the prepared skillet and press it evenly into the bottom. Lay marshmallow slices on top. Cover with remaining dough, flattening it evenly over the top. Sprinkle with remaining chocolate chunks.

Bake in the preheated oven for exactly 10 minutes.

While the cookie is baking, combine powdered sugar and almond milk in a large mixing bowl. Whisk icing to a drizzling consistency.

Remove cookie from the oven and cool slightly. Drizzle icing with a spoon over top of the cookie and top with crushed candy cane.