Preheat the oven to 180C and grease and line a 9x9in tin.
First make the brownie: Mix together the flour, Natvia, cacao and baking powder in a bowl. Whisk the peanut butter, apple puree, vanilla, vinegar and salt then pour into the dry ingredients and beat together. Spread in the tin.
Next place all of the cookie dough ingredients (apart from the chocolate chunks) into a blender or food processor and blend well until quite thick and smooth.
Spoon this over the top and spread out to cover. Sprinkle over the dark chocolate chunks and push in slightly then bake for 25-30 minutes until golden and firmed up. Leave to cool then cut into pieces. Enjoy!