1. Preheat oven to 350ºF. Line an 8-inch square baking pan with heavy-duty aluminum foil, leaving a 1-inch overhang on at least 2 sides. Mist with cooking spray.
2. In a small bowl, whisk flour, baking powder and salt. Melt butter in a medium saucepan over low heat. Remove from heat and whisk in brown sugar until no lumps remain. Whisk in egg and vanilla. Stir in flour mixture until just incorporated. Stir in peanuts and pretzels.
3. Pour batter into baking pan and smooth with a spatula. Drop spoonfuls of dulce de leche on top; swirl decoratively with a knife. Bake blondies until lightly browned and a toothpick inserted into a solid part comes out clean, about 30 minutes. Let cool fully in pan on a wire rack.
4. Grasping overhanging foil on either side of pan, lift out blondies and place on a cutting board. Carefully peel off foil before cutting into 16 squares. Serve.