Preheat oven to 325 degrees. Lightly spray a 12-inch cast iron skillet with nonstick cooking spray and set aside. Line a baking sheet with parchment paper and set aside.
In a large bowl, mix butter-flavored shortening with water, canola oil, sugar, and brown sugar together until light and fluffy. Scrape the sides of the bowl and mix again.
Add in the eggs and vanilla and stir again.
Measure out all the dry ingredients into the bowl and stir slowly until just incorporated. Scrape the bowl really well and stir again briefly to mix.
Divide cookie dough in half and spread one half in the bottom of the cast iron skillet. Gently press marshmallow halves and chocolate into the cookie dough. Take the remaining half of the cookie dough and divide in half again.
With one of the halves of dough, press between your hands to flatten slightly and place overtop the marshmallows and chocolate until mostly covered.
Top with remaining chocolate pieces and bake 22-25 minutes. Remove from oven and cool 10-20 minutes.
With remaining cookie dough, scoop into 10 small cookies and bake at 350 degrees for 6-7 minutes.
To serve, cut skillet cookie into pieces and top with ice cream and chocolate syrup. Crumble baked cookies into crumbs over top each piece and serve.