Line a 10 by 16 inch jelly roll pan with wax paper.
Microwave the milk chocolate chips at half power for one minute. Stir and if it is not completely melted, microwave for another thirty seconds. Spread over the wax paper.
Microwave the peanut butter chips and the butter together at half power for one minute. Stir to smooth it. Put several spoonfuls over the bark in different spots and then swirl it in.
Working quickly, spread the pretzels over the chocolate mixture. Push it down into the bark.
Place in the refrigerator for 30 minutes or until completely hard. Then break apart.
Store in an airtight container for up to two weeks.