Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" Square Pan.
Make the oat cookie layer by combining all the ingredients in a large bowl and mixing to form a cookie dough. Press 3/4 of this into the lined baking tin (keep back about 1/4). Ensure the layer fully covers the base of the pan in an even layer. Bake for 10 minutes.
Meanwhile, make the brownie layer. Start by melting the chocolate: break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn’t burn.
Combine the melted butter, granulated sugar and light brown sugar in a large bowl/bowl of a stand mixer - on a medium speed for 3-5 minutes.
Add in the eggs and vanilla and mix again. Then, checking first that the chocolate isn't so hot as to cook the eggs, add it in and keep mixing.
Add the flour and cocoa powder and fold in gently. Pour this mixture over the baked cookie layer (assuming it has cooled just a little - you will find it to be a bit puffy when it first comes out the oven but it will settle).
Sprinkle over the remaining oat cookie mixture and bake for 30 minutes. Let cool on a wire rack before slicing and serving!
