Grease a 9x13-inch pan and set aside.
Add 6 cups of marshmallows, the butter, corn syrup, cocoa powder and salt to a large saucepan.
8 cups mini marshmallows, 6 tablespoons butter, 1/4 cup corn syrup, 3 tablespoons cocoa powder, 1/4 teaspoon salt
Place over medium heat and stir frequently until the marshmallows are melted and smooth.
Remove from the heat and stir in the chocolate chips and vanilla until the chocolate is completely melted.
1 cup chocolate chips, 1 teaspoon vanilla extract
Stir in the cereal until well-coated.
8 cups Rice Krispies, Cocoa Krispies or Cocoa Pebbles
Stir in the remaining marshmallows.
Press into the prepared pan and let cool.
Top with melted chocolate, if desired, and any toppings you'd like!
additional chocolate chips and/or sprinkles for topping
Cut into squares and serve.