Heat oven to 350°. Line the bottom of a 9×13” baking pan with parchment paper.
Crush pretzels in food processor. Place in a medium sized bowl and add sugar and melted butter. Stir and press into the bottom of your pan. Bake for 8 minutes.
Meanwhile, melt butter and peanut butter chips in a medium saucepan over low heat, stirring constantly, just until melted. Remove from heat and cool slightly.
Stir remaining brownie ingredients together in a large bowl. Stir in peanut butter mixture. Spread over pretzel crust and bake about 30 minutes, until a toothpick comes out clean. Cool completely.
Make the frosting: Beat butter and peanut butter with a hand mixer. Beat in powdered sugar, vanilla, and milk. Mix until smooth and spread over cooled brownies. Cut into bars.