Preheat your oven to 350°F (175°C).
Grease your oven-safe skillet with a bit of canola oil or cooking spray.
In a large mixing bowl, combine the butter-flavored shortening, canola oil, granulated sugar, and brown sugar. Whisk until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
In a separate bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt. Gradually add dry ingredients to wet mixture, stirring until just combined.
Gently fold in the halved marshmallows and chopped chocolate bars.
Spread the cookie dough evenly in the prepared skillet. Bake for 25-30 minutes, or until edges are set and center is slightly soft.
Allow the skillet cookie to cool for about 10 minutes before serving. Top with vanilla ice cream and drizzle with chocolate syrup if desired.
