In a bowl, sift the flour, cocoa, corn flour with the baking soda. Add and stir the salt as well.
In another bowl, add the butter and the 2 types of sugar. With a spatula, stir until the ingredients become one and fluffy. Add the egg and the vanilla and continue stirring.
Add the flour mixture little-by-little into the butter mixture. Then, mix the chocolate chips inside. With an ice cream scoop take dough and knead into a ball of about 40g. Put the balls in a pan with non-stick baking paper. Place them into the freezer for 1 hour. Remove them and place them on a pan with non-stick baking paper, leaving about 3cm between them. Let them defrost for 15 minutes and then cook at 180°C for 7 minutes or until their edges become golden-brown. Let them cool down and then remove from the pan, because when we remove the pan from the oven, they will be too soft.
Melt the marshmallow in the microwave oven for ½ - 1 minute. Remove and stir with a spoon. Take some of the mixture every time and open to shape a web. Stick it onto the cookies.