Preheat oven to 350° F. Line bakng sheet with parchment paper. Spread pecans in an even layer on baking sheet. Bake for 8-10 minutes until nuts are fragrant and toasted. Once cooled to room temperature, roughly chop nuts. Set aside.
IIn a heavy bottomed sauce pan, melt butter over medium heat. Once the butter has melted, continue to cook. Swirl pan occasionally to ensure even heating. Mixture will bubble and foam as it cooks. The bubbling will be vigorous at the beginning but it will subside. Continue to cook until butter smells nutty. The mild solids in the butter will turn rich brown in color. Remove from heat and pour into a heat-safe bowl. Be sure to scrape out as much of the browned milk solids bits. Let butter cool for 10 minutes.
Line an 8-inch square baking pan with parchment paper, allowing for an overhang on all four sides (this will aid in unmolding after baking). Lightly grease and set aside.
In a large bowl, combine brown sugar, salt, cooled brown butter, and oil. Beat together until sugar is well moistened. Add egg and vanilla. Continue to mix until smooth and creamy.
In another bowl, whisk together flour, baking soda, and baking powder. Add flour mixture to butter mixture and beat to combine. Batter will be rather thick. Fold in cooled chopped pecans until well distributed.
Transfer batter to prepared pan and press down into even layer. Bake for 25-30 minutes until toothpick inserted in center of pan comes out clean. Allow to cool in pan for 15 minutes. Lift out blondies using parchment overhang. Transfer to cooling rack to cool completely.Alternatively, store blondiesin the fridge until chilled. Once cool, slice into desried serving pieces.