Preheat the oven to 180ºC / 160ºC Fan / Gas 4 / 350ºC F Melt the butter in the microwave and add it to a large mixing bowl with the brown sugar and vanilla bean paste. Beat together using an electric handheld whisk or a wooden spoon.
Mix the cornstarch with the cold water until it's dissolved and pour it into the bowl, mixing everything together until it's combined.
Sieve the flour, baking powder and salt into the bowl and beat until you can no longer see streaks of flour. The cookie dough will be soft and pliable.
Using your hands take pieces of cookie dough (I use an ice cream scoop to measure the dough) Roll the dough in your hands into a ball and then use your fingers to make it flatter on the bottom and more dome-like at the top.
Arrange the cookie dough balls on two baking trays. There will be six to a tray. Leave plenty of space for when they spread and become cookie-shaped in the oven.
Bake in the centre of the oven for 12 mins. You'll need to do this in two batches unless you have a double oven. The cookies are baked when they feel firm on top. Once baked remove from the oven and transfer to a cooling rack.
To make the caramel, put the coconut milk, brown sugar and vanilla bean paste into a medium-sized saucepan. Bring it to the boil stirring continuously over a high heat. I use a small handheld whisk to stir caramel.
When the contents of the pan come to a boil turn the heat right down and let it simmer until it's reduced by about half. This will take around 20 minutes. You must watch the pan like a hawk otherwise it will boil over! You may need to periodically remove the pan from the heat if the sauce is bubbling too close to the top of the pan.
Once the caramel has reduced and is very dark brown in colour, set the pan aside to cool. The sauce will thicken as it cools.
When the caramel has cooled pour three tablespoons of the sauce over the popcorn and marshmallows and mix together. Top the cookies with piles of popcorn and marshmallows then pour the caramel sauce on top of the cookies to glue the topping on. Put the cookies aside for around 30 minutes until the caramel has set.