Step 1: In a light-colored saucepan, melt the unsalted butter over medium heat. Continue cooking, stirring occasionally, until the butter turns a golden brown color and nutty aroma fills the air. Immediately remove from heat and pour into a heatproof bowl to cool slightly.
Step 2: In a large bowl, combine the cooled brown butter with the light brown sugar and granulated sugar. Mix well until creamy. Beat in the eggs one at a time, then stir in the vanilla extract until well combined.
Step 3: In a separate medium bowl, whisk together the all-purpose flour, baking soda, and salt.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Stir in the old-fashioned rolled oats and toasted chopped pecans until evenly distributed.
Step 5: Cover the cookie dough and chill in the refrigerator for at least 30 minutes, or up to 2 hours, for best results and to prevent spreading.
Step 6: Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
Step 7: Scoop rounded tablespoons of dough onto the prepared baking sheets, leaving about 2 inches between each cookie. Bake for 10-12 minutes, or until the edges are golden brown and the centers are still slightly soft.
Step 8: Remove from the oven and let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.