In a plastic bag with the flat side of a mallet or a hammer, smash the pretzels until they are broken but not pulverized.
Line a baking sheet with a silicone nonstick liner or spray it with nonstick cooking spray.
In a medium to large microwave-safe bowl, melt the chocolate in the microwave, stirring in 30 second increments until it is smooth (alternatively, melt it on the stovetop in a double boiler).
Stir in the marshmallows and pretzels, then, using two spoons, scoop spoonfuls of the mixture onto the lined baking sheet.
Refrigerate the clusters for about 15 minutes until they are solid, and serve immediately or store them in an air-tight container for up to a week.