Preheat the oven to 350°F. Lightly grease a 9"x13" baking pan, line the bottom with parchment paper and grease that lightly as well.
Melt butter in a large saucepan on medium heat. Cook stirring occasionally until the butter starts to brown lightly and smells very fragrant.
Add the brown sugar and stir with a wooden spoon. Make sure to scrape the brown bits from the bottom and cook until the sugar has melted and mixture thickens.
Turn off the heat and the sugar. Stir until combined. Let the mixture cool slightly, about 8~10 minutes.
While the sugar mixture is cooling, whisk flour, baking powder and salt.
Once the sugar mixture is slightly cooled, add a third of the flour mixture and mix until combined. Add half of the beaten eggs mixture - stir until combined. Continue with third of the flour mixture and the remaining egg mixture. Finally add the remaining flour and mix until smooth.
Pour the batter into the prepared baking pan and sprinkle the chocolate chips or chopped nuts on top.
Bake for 20~25 minutes or until the edges are browned and the center is firm to touch. Do not over-bake.
Score the blondies while still warm and let them cool completely in the pan. Once cooled, slice into pieces and serve. Leftovers stay fresh for up to 4~6 days well covered.