Line a rimmed cookie sheet with parchment or waxed paper.
Melt the chocolate in a double boiler over simmering water until smooth, or melt in the microwave in a 2 quart glass dish. Microwave on high(100%) for 1 minute, stir, and microwave on high for 1 minute more. If necessary, microwave for 10 seconds more, and stir until the chocolate is melted and smooth. Stir in the pretzel pieces, nuts and M&M's. Stir will with a flexible spatula.
Spoon onto the prepared cookie sheet, spreading the mixure out so that the ingredients are relatively evenly distributed. Allow bark to cool for several hours or refigerate for about 45 minutes, until firm.
Break the bark into irregular pieces, like peanut brittle. Store in cookie tins, or package in Chinese food take-out cartons lined with waxed paper