Brown the butter: In a medium saucepan over medium heat, melt the butter, stirring constantly until it foams and turns golden brown with a nutty aroma. Remove from heat and let cool slightly.
Mix wet ingredients: In a large bowl, combine browned butter, brown sugar, and granulated sugar. Whisk until smooth.
Add egg and vanilla: Beat in the egg and vanilla extract until fully incorporated.
Combine dry ingredients: In a separate bowl, whisk flour, baking soda, salt, and cinnamon (if using).
Combine wet and dry: Stir the dry ingredients into the wet mixture until just combined.
Fold in oats: Add the oats and mix until evenly distributed.
Add mix-ins: Fold in chopped pecans, dates, and chocolate chunks.
Chill the dough: Cover and refrigerate for 30 minutes to allow the flavors to develop and prevent spreading.
Bake: Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper. Scoop 1.5 tablespoon portions of dough onto the sheets, spacing 2 inches apart. Bake for 11–13 minutes, until edges are golden and centers look slightly underbaked.
Cool: Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.