Use a whisk to cream the butter and sugar until it's very soft and creamy.
Add the egg and vanilla and whisk again.
Add the flour, baking powder, salt, and mix using a spatula.
Add the chocolate and marshmallows and mix them in as well.
Form balls of dough between your hands, making sure that the marshmallows are covered in dough. If they stick out they melt and disappear in the oven.
Flatten the dough balls slightly and place them on a baking tray lined with baking paper. Repeat with all the cookies, leaving space between them because they expand a bit in the oven.
Bake the cookies in a preheated oven at 180°C/350°F for 12-15 minutes or until they're just cooked, to keep them soft. You don't want them to be golden at all.
Because of the marshmallows, these cookies can sometimes be a bit wonky in shape. As soon as they come out of the oven, you can surround them with a cookie cutter and roll them inside it to even out the edges. You have to do this as soon as they come out of the oven.
Right afterwards, place extra chocolate chunks or chips on top and push them in slightly. This makes chocolate pools. Finish them with flaky salt.