Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
Chop walnuts into small pieces.
In a bowl, whisk brown sugar, egg, and vanilla until combined.
Pour in melted butter and whisk until glossy.
Add oats, rolled oats, baking powder, and salt. Mix until thick.
Fold in chopped walnuts.
Stir in chocolate chips if using.
Shape into 12 balls (about 2 tbsp each) and lightly flatten into thick discs.
Bake 10–12 minutes, until edges are set and lightly golden.
Cool 10 minutes on the pan, then transfer to a rack to finish cooling.
