1. Preheat oven to 350°F. Line a 9-by-13-inch baking pan with parchment.
2. In a bowl, mix flour, baking powder and salt. Using an electric mixer, beat butter and sugar until light. Beat in eggs, one at a time, then vanilla. Reduce speed to low; beat in flour mixture. Stir in chocolate chips and 1 cup pretzels. Spread evenly in baking pan. Bake until a toothpick inserted into center comes out clean, 25 to 30 minutes.
3. In a bowl set over a pan of simmering water, melt chocolate with peanut butter, stirring until smooth. Spread mixture evenly over blondies. Sprinkle with remaining crushed pretzels. Let cool before cutting into squares.