Begin by lining a cookie sheet or baking pan with parchment paper. Set aside.
Grab two small microwave-safe glass bowls and add 1 tablespoon of butter or margarine to each bowl.
Pour the milk chocolate chips into one bowl. Then, pour the peanut butter chips into the other bowl.
Place each bowl, one at a time, into the microwave. Heat each bowl (separately) for 1 minute, then remove and stir. Place back in microwave if needed for approximately 30 to 60 seconds more. Be sure to pause the microwave every 15 seconds to check the mixture and stir until it’s smooth.
Next, grab two large spoons. Place one in each bowl. Then, start to drizzle each flavor on the parchment randomly, it doesn't need to be neat as it's being swirled anyway. Once you have all the milk chocolate and peanut butter mixture on the pan you can use a knife or offset spatula, to swirl the mixtures together. Run the utensil through the chocolate and peanut butter to make any pattern you prefer.
Randomly scatter the Reese's pieces over the bark.
Place the cookie sheet or baking pan in the fridge for about 30 minutes.
Remove the bark from the refrigerator and break it into pieces by hand, or use knife to cut it if you prefer.
Serve. Or store it for later.