With a wooden spoon or mixer, mix together the butter and sugar. Add the cocoa powder, salt, vanilla and flour. Rub the ingredients together with the tips of your fingers until it comes together into a dough. Roll it into a ball, wrap in cling film and rest in the fridge for half an hour.

Meanwhile, prepare your silicone dome moulds with chocolate. Melt 300g of your chocolate either in the microwave in short bursts or in a bowl over a pan of simmering water, and then spoon around a tablespoon of melted chocolate into each mould. Use the back of the spoon to spread it around each dome, ensuring that you bring the chocolate right up to the edge.

Put domes in the fridge for a few minutes to allow the thinnest part of the chocolate to harden, then using a pastry brush, spread any pooled chocolate in the centre around the dome. Return to the fridge until ready to assemble.

Remove dough from the fridge and roll onto a lightly floured worktop until it is around ¾ cm thick. Use a small circular cutter to cut out 12 biscuits. Place the biscuit circles on a lined baking tray and return to the fridge for around another 20 minutes. Meanwhile pre-heat the oven to 190ºC (170ºC Fan).

To make the coffee marshmallows, add the egg whites, coffee powder, sugar, salt and golden syrup in a large mixing bowl, giving it a light whisk. Place the bowl over a pan of simmering water without letting the bottom touch the water and let the mixture reach 71ºC so the eggs are pasteurised, ensuring you stir/whisk so that you don't let the mixture cook and get scrambled eggs. Once it has reached this temperature, transfer to a standing mixer, or use a hand held mixer to whisk for around 7 or 8 minutes, until the marshmallow is completely cool and it has reached stiff peaks. Once it is ready, transfer to a piping bag, ready to assemble!

Remove biscuits from the fridge and bake for around 10minutes, or until slightly darker around the edges and allow them to cook fully. They will seem very soft when they come out the oven but the with crisp up as they cool.

To assemble, melt the remaining 100g of chocolate and leave to cool slightly so it is not too hot. Pipe the coffee marshmallow into the chocolate moulds, leaving enough room for the biscuits. Carefully place a biscuit on top of the marshmallow in each mould. Cover each biscuit in a layer of chocolate, and fill in any gaps so that the marshmallow and biscuit are completely sealed in the moulds. Try to ensure there is not too much excess chocolate around the sides of the moulds as this will make them neater when you pop them out! Place in the fridge until set. Once set, carefully pop the teacakes out of their moulds and tuck in!