Preheat the oven to 350°F (175°C).
In a mixing bowl, beat together the butter and dark brown sugar until light and fluffy.
Add the egg and vanilla extract, mixing until well combined.
In a separate bowl, whisk together the flour, cocoa powder, salt, and baking soda.
Gradually add the dry mixture to the wet ingredients, stirring until just combined.
Fold in the rolled oats and dark chocolate chips until evenly distributed.
Scoop rounded balls of dough onto a lined baking sheet, leaving space between each cookie.
Bake for 10-12 minutes, until the edges are set and centers look slightly underbaked.
Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.
