Preheat the oven to 150°C (300°F) and line a baking tray with baking paper.
Cream the butter and icing sugar together until it is light and fluffy.
Add in the vanilla extract, cornflour and flour and mix well.
Roll out the dough to a 5mm thickness and using a small, round cookie cutter, cut out the cookies.
Alternatively, roll the dough into a large sausage shape and chop 5mm thick slices from that.
Prick each one with a fork and bake for 20-25 minutes. They should just begin to be getting a pale golden colour.
Once they have cooled, melt 150g of milk or dark chocolate and dip one side of each cookie in the chocolate so there is a even layer on every cookie.
Place these in the fridge to set.
Marshmallow Creme;
Place the sugar, water and glucose syrup in a saucepan on a high heat and bring to the boil.
Boil the syrup for at least 5 minutes.
While the sugar mixture is boiling, beat the egg whites, vinegar and salt together until soft peaks form.
With the beaters still on, pour in a few tablespoons of the sugar mixture into the egg whites and beat until fully combined.
Pour in the remaining sugar and beat on high for around 5-7 minutes.
The mixture will be glossy, thick and nearly triple in size.
Beat in the vanilla and set aside to cool.
Using a piping bag, pipe the marshmallow onto each shortbread cookie, putting pressure at the base to make the marshmallow spread before pulling the piping bag up to create a cone like shape.
Place these in the fridge for an hour.
Melt the remaining chocolate and butter together and allow to come to room temperature.
Place the cookies on a cooling rack with a baking tin underneath to catch any chocolate.
Pour the chocolate over the marshmallow cookies.
Scoop up the chocolate that falls into the baking tin and repeat until all the cookies are covered.
Place them in the fridge to set.
Notes
These freeze well. Halve the recipe for less. Milk or dark chocolate works. You could even try white chocolate for something different! Cooling the chocolate is important otherwise the hot chocolate would melt the marshmallow creme