Preheat oven to 200C/180C Fan/Gas Mark 6. Line an 8″ Square Pan.
In a large bowl, cream together the butter and sugars for 5 minutes with an electric mix (or in a stand mixer). Add the egg and vanilla and combine again.
Add the flour, oats, baking powder and chocolate chunks. Fold together to form a soft cookie dough.
Spread this mixture into the lined tin, it can be a little sticky. Sprinkle any remaining chocolate on top and bake for 30 minutes.
Let cool entirely on a wire rack before slicing and serving (otherwise they are prone to falling apart!).