Preheat oven to 375° F. Lightly mist a 12-cup muffin tin with non-stick cooking spray. In a small bowl mix flour, baking powder and salt. Stir to combine.
Melt butter in a medium saucepan. Remove from heat and stir in brown sugar. Let cool slightly then stir in egg and butter until smooth. Stir in flour mixture and then chocolate chips and nuts (if using).
Portion out about ¼ cup of batter into each muffin cup. Smooth tops with the back of a spoon. Bake until just set, 15 - 17 minutes. Let cool completely in a pan on a wire rack. To remove from the pan, gently run the blade of a butter knife around the edge and lift out.