Place the butter and sugars in a freestanding mixer or a bowl using a hand-held electric mixer and beat until light and fluffy. Break in the eggs, one at a time and then add the vanilla extract.
Sift in the flour, cocoa powder and baking powder and then using a large metal spoon fold them in until well combined. Add the chocolate and mix again until just combined.
Bring the dough together and split in half. Place each half on a piece of clingfilm and roll up to form a thick sausage shape, twisting the ends to secure. Place in the fridge overnight to rest and firm up.
To bake, preheat the oven to 350°F (180°C), Gas mark 4. Unwrap each piece of dough and cut each one into 12 even slices. Arrange on parchment lined large baking sheets well spaced apart and bake for 10-12 minutes or until golden brown and slightly pale in the centre. Leave the cookies to cool for a couple of minutes on the baking sheets and then transfer to a wire rack and set aside until cool.
To serve, arrange the double chocolate chip cookies on a large plate and hand around to guests.