NYC Cookie Dough: in a bowl, beat together the butter and sugars until pale and fluffy. Crack in one egg at a time and mix well until combined. Add in the vanilla extract, flour, baking powder, bicarbonate soda, salt and chocolate chips, and mix to combine. Divide the dough into 8 portions. Refrigerate for at least 10 minutes
Assemble: on a lined baking pan, space out 8 Graham/Digestive biscuits evenly. Top each biscuit with a chocolate square and marshmallow. Working with one cookie at a time - take a portion of the NYC cookie dough and roll into a log in-between your hands. Wrap the log of cookie dough around the marshmallow and use your fingers to press the dough into the marshmallow and seal the cookie to the biscuit base. Repeat for all the remaining cookies.
Refrigerate the cookies for at least 30 minutes, ideally 1 hour.
Pre-heat your oven to 170C/340F and bake the chilled cookies for 12-15 minutes. Let the cookies rest for 10 minutes before enjoying them warm.