Preheat the oven to 180°C/160℃ fan/350°F/gas mark 4. Grease a 6 in cast iron skillet.
Beat the butter and sugars together with an electric mixer until pale and fluffy. Add the milk and vanilla extract and beat again to combine.
55 g (¼ cup) vegan block butter, 40 g (3 ½ Tablespoons) light brown soft sugar, 35 g (scant 3 Tablespoons) caster or granulated sugar, ½ teaspoon vanilla extract, 1 Tablespoon non-dairy milk
Add the flour and baking powder and mix until it forms a dough, then tip in the chocolate and mix until evenly distributed.
90 g (¾ cup) plain flour, ¼ teaspoon baking powder, 75 g (2.75 oz) dark chocolate, chopped into chunks (or use chocolate chips)
Spread about half of the mixture across the base of the skillet, pressing it out with your fingers, so that it is completely covered in a thin layer of dough.
Spread the nutella on top of the dough using the back of a spoon, leaving a 1.5cm border of cookie dough around the edge.
3 Tablespoons vegan nutella/chocolate hazelnut spreaad
Use your fingers to flatten pieces of the remaining cookie dough and press them on top of the mixture in the skillet, try to press them together to create an even layer and make sure that the nutella is covered.
Bake the cookie for 15-20 minutes until golden and crispy around the edges, the middle should still be a little soft but it will firm up as it cools. Allow the cookie to cool for at least 15 minutes before serving, topped with a dollop of ice cream and a drizzle of warm nutella.