Preheat oven to 350 degrees. Spray a large (12 inch) cast iron skillet (or 9x13 inch pan) with non-stick spray.In a large bowl, mix together the cake mix, milk, melted butter, and eggs until well combined. Stir in sprinklesPour cake into skillet. Drop the strawberry jam by Tablespoon fulls onto prepared cake, and swirl through with a knife.Bake cake in preheated oven for 34-38 minutes.Allow cake to cool before frosting.Frosting:Whisk together the powdered sugar, jam, vanilla, and milk until well combined. Add frosting to a ziploc bag, and cut about ¼ inch off of the corner (or use a piping bag), and frost cake as desired.