1. Preheat the oven to 180 degrees Celsius and line an 8×8 square baking tin with baking paper coving both the bottom and sides.
2. If using dark chocolate, roughly chop the bar into small chunks and set aside.
3. For the cookie dough layer, first melt the vegan butter in a small pan over a low-medium heat and once melted, add in the brown sugar. Stir to combine until the sugar has melted, then pour in the oat milk and vanilla extract and stir through. Remove from the heat and leave to cool.
4. In a large mixing bowl, add the flour, baking powder and salt, and lightly whisk together. Make a well in the centre and pour in the cooled, wet mixture, then stir to combine, using a whisk if necessary but being careful not to over mix.
5. Fold in the chocolate chunks or chips, then pour the cookie dough mixture into the lined baking tin, spread evenly and place in the fridge.
6. For the brownie batter, melt the vegan butter in a small pan over a low-medium heat and once melted, add in the brown sugar and stir to combine until the sugar has melted. Next, add in the Mr Organic Dairy Free Organic Chocolate & Hazelnut Spread and oat milk, and stir to combine, then remove from the heat and allow to cool.
7. In a large mixing bowl, combine the flour, cocoa powder, baking powder and salt together with a whisk. Make a well, then pour in the cooled, wet mixture and stir to combine, using a whisk if necessary but being careful not to over mix.
8. Pour the brownie batter over the cookie dough layer and even out with a spatula.
9. Place in the oven to bake for 25-27 minutes. A toothpick inserted into the centre of the brownie should come out not quite clean with a few crumbs for a gooey texture.
10. Allow for the brownie to cool completely and once fully cool, place in the fridge to set for 2-3 hours for the ultimate gooey texture. Slice and enjoy!.