Preheat the oven to 170℃ and line a baking tray with baking paper. We start by crumbling the cookie dough into a bowl and add the cocoa powder. Start mixing this with your hands in the bowl and then turn out the contents onto a clean work surface. Knead the dough well to ensure all the cocoa powder is mixed through. This might take a few minutes so be patient.

Roll the dough out between two sheets of cling wrap and a rolling pin to 1/8th of an inch in thickness. We want this fairly thin as it will be sandwiched with ganache filing. Cut into squares and transfer to the lined baking tray. Bake at 170℃ for 6-8 mins until done. Let them cool down.

To make the ganache filing we need to heat the heavy cream in the microwave for 40-50 seconds until steaming hot. Add the chopped dark chocolate into the cream and let it sit for a minutes before stirring to combine. Add the butter and stir again. Let this set for 5 mins in the fridge before transferring to a piping bag.

Pipe the ganache into a now cooled cookies and sandwich with another cookie. Drizzle with more melted chocolate on top and sprinkle over sea salt. Serve with a glass of milk.