Use the extra butter to coat the bottom and sides of the 9 x 13-inch pan. Set aside.
In a large pan, melt the butter over low heat. Add the cocoa powder and mix until well combined. Add the marshmallows. Stir until the marshmallows are starting to melt. Turn off the heat. Continue to stir until the marshmallows are completely melted. Add the vanilla and stir.
Add the cereal and stir until it is coated with the chocolate mixture.
Place the mixture into the buttered pan. Place a sheet of waxed or parchment paper on top and press the cereal mixture until it is evenly spread. Cut into squares or use a cookie cutter to cut shapes. If using a cookie cutter, reform any leftover mixture and continue cutting shapes until all the cereal mixture is cut.
Sprinkle the chocolate chips over the mixture and use the parchment paper to press them into it.
Notes
Troubleshooting
The most common complaint with any crispy treats is that they turn out too hard. This usually happens because the heat was too high when melting the marshmallows. Be sure to use low heat and remove the mixture from the heat before all the marshmallows are melted. Then stir until all the marshmallows are melted.
Substitution
You may substitute large marshmallows for miniature marshmallows. Use the same amount of ounces listed in the recipe. You can also cut them into two pieces to make them melt faster.
Gluten-Free
Use gluten-free crispy rice cereal to make a gluten-free treat.