Preheat the oven to 190C / 170C Fan / Gas Mark 5. Line 2 baking trays with non-stick parchment paper. 

In a large bowl, beat together the butter and the sugars until light and fluffy. Add in the eggs, one at a time, beating after each addition. Stir in the vanilla extract. 

Sieve in the flour, bicarbonate of soda and salt. Mix until it forms a soft dough. Stir in the chocolate chips. 

Take 1 tablespoon of cookie dough, place on the tray and flatten. Top with 1 square of chocolate and 1 heart marshmallow. Take 1 more tablespoon of cookie dough, flatten and place on top of the marshmallow. Use your hands to mould the cookie dough around the marshmallow until it is fully covered. 

Place in the freezer for 30 minutes or these can be made and frozen the day before. 

Leaving space between each cookie, bake for 15-18 minutes, or until the cookie dough is golden. Remove from the oven. 

As soon as they come out of the oven, top each cookie with some milk and white chocolate hearts. The heat of the cookie will melt them into the top. 

Allow to cool for 5 minutes before serving. They taste best when warm.