To make the pie shell, preheat the oven to 160C. Put the biscuits and pretzels in a food processor and whiz for 30 seconds. Add the butter and sugar and process again until fully mixed. Line a 16-centimetre foil pie dish with the mix, pressing it up the edges and firmly into the base. Bake in the oven for about 10 minutes, then cool the shell.

Place the milk and cream in a heavy-based saucepan and heat until just about to simmer. Whisk sugar and yolks together, pour in warm cream mix, then return to the saucepan. Stir over a medium heat until the mixture thickens slightly and coats the back of a spoon. Remove from the heat. Add the chocolate, stirring until melted and well combined. Fold in the semi-whipped cream before the mix cools and pour into a container. Cover and refrigerate for  one hour.

When the soft chocolate is cool and firm it is ready to go into the pie shell. Spoon it in and smooth the top with the back of a spoon. Put a two-centimetre nozzle on a piping bag and fill it with half the unset marshmallow mix. Pipe it in a circular shape on top of the chocolate, starting from the outside and finishing in the centre.

If you have a blowtorch, scorch the top of the tart quickly without melting the marshmallow. Otherwise, place pie straight into the fridge for an hour before serving.