Preheat the oven to 350 degrees F. Line a 9x13 inch pan with foil, leaving enough to hand over two opposite edges. Spray lightly with cooking spray.
In a large bowl, use an electric mixer to combine the butter and sugars until smooth. Add the egg, salt and vanilla and mix well. Add the baking soda and flour, and mix just until incorporated. Stir in the broken pretzels, peanut butter chips and M&M's.
Spread the batter evenly into the prepared pan. Gently press the pretzel twists into the top, 4 in each row for 6 rows. Press some extra M&M's on top around the pretzels for a more festive look.
Bake for 20 minutes, or until a toothpick inserted into the center of the blondies comes out clean. Allow to cool completely before cutting into 24 pieces. Store in a sealed container.