Butter a 9x13 dish, set aside. Melt butter in a large non-stick saucepan over medium-low heat. Stir in marshmallows and cook, stirring constantly until marshmallows have melted. Remove from heat and stir in 1/2 cup peanut butter and vanilla.
Pour Rice Krispies Cereal and 1 1/2 cups coarsely crushed pretzel pieces in to melted marshmallow mixture and gently fold mixture until cereal and pretzels are evenly coated. Spread mixture evenly into prepared 9x13 dish and spread into an even layer with a butter spatula.
Melt chocolate chips with 2 Tbsp peanut butter in a microwave safe bowl, in 30 second intervals stirring after each interval until melted and smooth. Pour melted chocolate mixture into a piping or ziploc bag, snip the tip of the end and drizzle mixture evenly over Rice Krispie Treats.
Sprinkle evenly with 1/4 cup coarsely crushed pretzel pieces, enjoy immediately with melted chocolate topping or cover and refrigerate until set for a set chocolate topping.