Place an oven rack in the center position and preheat the oven to 350 degrees F. Grease an 8x8 pan.
In a large bowl, whisk together the melted butter, egg, vanilla and brown sugar until smooth. In a separate bowl, whisk together the flour, baking soda, baking powder, and salt. Whisk the dry ingredients into the wet until smooth, then stir in the peanut butter chips and chocolate covered pretzels.
Pour into the greased pan, spreading the mixture evenly. Bake for 20-25 minutes or until a toothpick comes out clean.
While the blondies are cooling, place the bittersweet chocolate in a microwave-safe bowl. Heat in 15-20 increments, stirring every time, until the chocolate is smooth and shiny.
Once the blondies have partially cooled (they don't have to be completely at room temperature), brush a thin layer of chocolate on top, then sprinkle evenly with more chopped chocolate pretzels.
Let the bars finish cooling as the chocolate topping sets. The chocolate will firm back up, sealing the pretzel topping onto the bars. Slice into 9 or 16 even blondies and serve.
Leftovers will keep in an airtight container at room temperature for up to one week.