Begin by making the churros batter. Add the butter and sugar to a saucepan and bring to the boil. Pour in the salt and flour and briskly stir until a smooth looking dough forms on the end of the spoon. Remove from the heat and leave somewhere to cool for five minutes.
Add the vanilla essence and an egg and beat in until its combined, then repeat with the remaining eggs, beating them in one at a time until you're left with a smooth, slightly sticky paste.
Add this to a piping bag with a small serrated nozzle and pipe 6 small C shapes onto a lined baking sheet, then pop it in the freezer to harden.
Meanwhile, grease the back of a muffin tin and carefully pipe all the way up the sides to the middle to make a dome. Try to avoid holes!
Remove the handles from the freezer and stick them into the sides, then add the mugs to the freezer to set completely (roughly 4 hours).
Heat up a pan with the oil to 180°C/360°F.
Lever each churro mug off the muffin tin and fry one at a time until golden brown.
Let the oil drain off and mix the cinnamon and sugar together then roll the mugs in it.
Pour in hot chocolate and marshmallows and eat straight away!