Pre-heat the oven to 180ºc and line a 20cmx20cm square cake pan with parchment paper.
Mix the cookie dough according to recipe instructions then press half of the dough into the bottom of the lined cake pan.
Spread over the Nutella then add the remaining dough and smooth down.
Place the cake pan into the oven and allow to bake for 25-30 minutes. If the top of the bars brown too fast, cover loosely with a piece of foil.
Remove the baked bars from the oven and allow to cool in the pan for 10 minutes before transferring to a wire rack and allowing to cool for a further 10 minutes.
Carefully slice the bars (they might still be very gooey in the middle).