Heat oven to 350°F. Spray bottom and sides of 13x9-inch pan with cooking spray.
In large bowl, beat dry pudding and milk with whisk 2 minutes. Stir in cake mix and 2 cups of the miniature marshmallows until combined. Spread batter evenly in bottom of pan. Scatter chocolate chips evenly on top.
Bake 30 to 35 minutes or until toothpick inserted in center of cake comes out almost clean. Immediately sprinkle remaining 2 cups miniature marshmallows on top. Cool completely in pan, about 1 hour.
Sprinkle top lightly with dry hot chocolate mix just before serving. Cut into 4 rows by 3 rows.