Melt butter in a medium saucepan over medium heat. Stir constantly. Remove from heat immediately when the butter begins to turn brown (about 6 minutes). Pour browned butter into large mixing bowl. Stir in brown sugar. Let the mixture cool to room temperature.
Preheat oven to 350 degrees. Spray 8x8 square baking pan with cooking spray. Set aside.
To the cooled down butter/sugar mixture, stir in egg and vanilla extract. Add flour, baking powder, and salt. Stir in pretzel M&Ms.
Pour into prepared baking pan and bake for about 25 minutes, or until the top is lightly browned. Allow blondies to cool for at least 1 hour and cut into squares. Store in an airtight container for up to 10 days.