Preheat oven to 375°F. Mist a 12-cup muffin tin with nonstick cooking spray or line with paper muffin cups. In a small bowl, mix flour, baking powder and salt; stir with a fork to combine.
Melt butter in a medium saucepan over low heat. Remove from heat and stir in brown sugar. Let cool slightly, then stir in egg and vanilla until smooth. Stir in flour mixture, then chocolate chips and nuts, if using.
Portion out a little less than 1/4 cup batter into each muffin cup and smooth tops with a small spatula. Bake until just set, 15 to 17 minutes. Let blondies cool completely on a wire rack before removing from pan.