In a large saucepan, combine the butter, sugar, milk and cocoa. Bring to a boil over medium heat, stirring constantly. Cook and stir 1 minute., Remove from heat; stir in peanut butter and vanilla until blended. Stir in oats; let stand 5 minutes to cool. Fold in pretzels and M&M's. Drop mixture by tablespoonfuls onto waxed paper-lined baking sheets. Refrigerate until set.