Preheat your oven to 350°F (175°C) and grease a 9x9-inch baking dish.
In a large mixing bowl, combine the rolled oats, cocoa powder, brown sugar, baking powder, salt, and cinnamon (if using). Mix well.
In another bowl, whisk together the milk, eggs, and vanilla extract until fully combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Fold in the chocolate chips and nuts if using.
Transfer the mixture to the prepared baking dish and spread it evenly.
Bake in the preheated oven for 25-30 minutes, or until the center is set and the edges are slightly crispy.
Allow to cool for a few minutes before slicing into squares and serving.