In your large pot melt your butter on low-medium heat.
While your melting your butter, lightly grease your 9x13 and 8x8 - see note below
Once your butter is almost all the way melted add your marshmallows and vanilla and melt completely - stirring constantly.
While your marshmallows are melted measure out your rice krispies into your large bowl.
Once your marshmallows are completely melted stir in the rice krispies until throughly combined.
Now, stir in your chopped mars bats and mix until just combined - you dont want to want to fully melt the mars bars.
Store in an air tight container in the fridge for up to 1 week.
*Note the 8 x 8 pan is optional you can either make a thicker square in the 9 x 13 or reduce the rice krispies by 2 cups and don't add the extra marshmallows.
Recipe by Stacey's Recipes at https://www.staceysrecipes.com/mars-bar-rice-krispie-squares/