1. Unwrap all of the Mars bars, and chop them into pieces. Reserve 1/4 cup of the chopped bars for later.
2. In a large pot over medium heat, melt 1/2 cup of the butter until it’s just melted, then add the chopped Mars bars (minus the extra 1/4 cup). Stir the mixture until everything is smooth, then remove the pot from heat and add in the crisped rice cereal and remaining 1/4 cup of Mars bars. Stir everything with a wooden spoon until the cereal is completely coated in the chocolate. 
3. Empty the mixture into a greased 8″x 8″ pan, then gently spread it out. In a small heat safe bowl, microwave the remaining 1/4 cup unsalted butter and chocolate chips on high for 30 seconds, stirring until smooth.
4. Spread the chocolate topping over the top of the bars, then sprinkle a few extra Mars bars on top if desired. Let the bars harden for an hour in the fridge before slicing into 16 pieces.

Bars can be stored at room temperature, in an airtight container, for up to 1 week, or up to 2 weeks in the fridge. You can also freeze the bars for up to 2 months, thawing in the fridge overnight.