Brown the butter: Cut the butter into roughly tablespoon sized portions and add to a light colored frying pan (so you can see it brown easily). Melt the butter over medium heat. Continue to cook the butter about 5-7 minutes, stirring regularly, as it begins to froth and sizzle. Stir continuously as bits of solids brown in the bottom of the pan. Once the butter turns brown and begins to smell nutty, immediately remove it from heat and transfer to a bowl to cool partially. Cool for 10 minutes, until no longer hot.
In a stand mixer, combine browned butter, granulated sugar, and brown sugar at medium speed. Beat in the eggs, one at a time, and vanilla extract.
In a medium bowl, combine flour, baking soda, baking powder, and salt. Reduce the mixer speed to low. Gradually beat in flour mixture. Fold in the butterscotch chips, chocolate chips, and chopped pecans.
The dough will be fairly runny due to the melted browned butter, so chilling is necessary. Refrigerate the dough 1-2 hours before shaping.
When you're ready to shape dough, preheat the oven to 350℉. Spoon about 1½ tablespoons of dough onto a baking sheet, spacing 3 inches apart. Repeat until the cookie sheet is full. Spoon about 1½ tablespoons of dough onto a baking sheet, spacing 3 inches apart. Repeat until the cookie sheet is full.
Bake cookies for 10 to 12 minutes, until lightly golden but still soft. Allow cookies to cool partially on the baking sheet for 5 minutes before transferring to a cooling rack or wax paper to finish cooling.