Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the softened butter, brown sugar, and sugar. Beat until light and fluffy.
Add the eggs and egg yolk one at a time, mixing well after each addition. Stir in the vanilla extract.
In another bowl, whisk together the oat flour, cocoa powder, salt, and baking powder.
Gradually add the dry ingredients to the wet mixture. Stir until just combined.
Fold in the rolled oats and chocolate chips until evenly distributed.
Drop rounded balls of dough onto a parchment-lined baking sheet, leaving space for spreading. Bake for 10-12 minutes.
Allow the cookies to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
