Make the base: place the biscuits in a food processor and blitz until a fine crumb, then, with the blade running, pour the melted butter onto the biscuit crumbs. When combined, pour the crumb into a 20cm springform tin and using your hands, or a glass, push the crumb into the base and sides of the tin. Place into the fridge for at least 30 minutes.
Make the filling: Place the cream into a large bowl and add the sugar and coffee powder, then whisk until stiff peaks form. Add the caramel and fold in until well combined then spoon into the base, smoothing the top with the back of a spoon or an offset spatula. Place into the freezer for at least 4 hours.
You will need to remove the pie from the freezer 20 minutes before serving, which will giv you time to make the meringue.
Make the topping: in a small saucepan set over high heat, stir together 300g of the caster sugar, cream of tartar and water. Bring to a boil and without stirring, bring the mixture to 120 C on a sugar thermometer.
Meanwhile, place the egg whites in the bowl of a stand mixer fitted with the whisk attachment. Beat until medium peaks form then gradually add in remaining 100g caster sugar. Once combined, very carefully pour the hot sugar syrup into the mixture, making sure not to pour the syrup over the whisk. Whisk for 5 to 10 minutes until the bowl of the mixer has cooled to the touch and the meringue is thick and glossy then add the vanilla and whisk for a further minute..
Assemble the tart: Remove the pie from it’s tin and spoon the meringue mixture on top of the pie. Use a blowtorch to lightly toast the marshmallow topping before serving. Slice with a hot knife.