Put the sugar and water into a small saucepan and place over low heat. Stir to combine until the sugar has melted. Increase the heat to medium and simmer for about 5 minutes, until the sugar begins to brown at the edges. Gently swirl the pan so that the sugar cooks evenly, resisting the urge to stir. When the caramel has turned a clear amber – after about 1 minute – remove from the heat and pour on to a small parchment-lined tray. Tilt the tray as it sets to get a thin layer of caramel. Sprinkle all over with the ground coffee and set aside until completely cool and hard. Once cool, transfer to a food processor and blitz until roughly ground, or crack it by hand by folding the parchment in half then pressing down with the palm of your hands, repeating 4-5 times to crack the caramel. The praline pieces will range in size from powdery to about 0.5cm wide. Set aside until ready to use.

Preheat the oven to 160°C (fan). Cut two sheets of baking parchment to fit two large baking trays. Using the base of the tin in which you are going to freeze the parfait (a 20cm square or round tin works, or even a loaf tin really), trace around the base on to each of the sheets of parchment. Turn the paper over and place on the baking trays so that the side you have drawn on is facing down but still visible. Set aside: this will be the template for the meringue bases.

If starting with whole hazelnuts, spread them out on a baking tray and roast for about 15 minutes, or until the nuts are lightly brown. Remove from the oven and, once cool enough to handle, chop them finely into 0.5cm pieces. If starting with chopped nuts, just spread them out on a baking tray and roast for 7–8 minutes. Set aside until completely cool.

Sift the icing sugar and cocoa powder together into a bowl and set aside. Place the egg whites in the bowl of an electric mixer with the whisk attachment in place. Add the salt and cream of tartar and beat on a medium-high speed for about 2 minutes, until soft peaks form. Gradually add the caster sugar – a tablespoon at a time – and continue to beat for 2–3 minutes, until very stiff. Gently fold in the sifted icing sugar and cocoa powder and the hazelnuts.

Divide the meringue mixture in two and dollop on to the two traced sheets, smoothing it out with a small metal spatula to fit the traced shapes. It will spread slightly as it bakes, but you can trim it back so that it fits the tin snugly later.

Bake the meringue sheets for about 1 hour, or until the meringues are crisp and dry (the inside will still be a little bit soft). Check the meringues halfway through and rotate the trays. Once done, remove them from the oven and set aside to cool. Once the meringues are cool, trim the edges so that they fit neatly into the tin in which the parfait will be frozen.

Melt the chocolate on a double boiler or in a microwave (30-45 seconds should do the trick), then set aside to cool slightly. Brush the flat side of each of the meringue bases with the melted chocolate and immediately sprinkle the coffee praline all over the chocolate so that it has time to adhere to the chocolate before it sets. Set aside to dry while you make the parfait.

Place the cream in the bowl of an electric mixer with the whisk attachment in place. Beat until soft peaks form, then transfer to a container and store in the fridge until ready to use.

Place the egg yolks in a clean bowl of an electric mixer with the whisk attachment in place, in preparation for the next stage. Put the espresso coffee and sugar into a medium saucepan and place over a low heat, stirring until the sugar has dissolved. Increase the heat to medium-high and simmer the coffee syrup. In the meantime, begin whisking the egg yolks on a medium-high speed. Keep checking the coffee syrup with a sugar thermometer and when it has reached 118°C (this should take 10-15 minutes), pour the hot coffee syrup slowly down the rim of the bowl on to the beating yolks, which should be thick and creamy by now. (If you don’t have a thermometer, you can get a small bowl of cold water, and spoon a bit of the boiling sugar into it. The sugar is at the right temperature if you can pinch the sugar together with your fingers and roll it into a small, soft ball. If the syrup diffuses through the water, it’s not ready yet; let it boil for longer.) Continue to whisk at medium-high speed until the bowl is no longer warm to touch – 10–15 minutes – then add the coffee liqueur. Whisk to combine, then turn off the mixer.

Prepare the tin; while the yolks are whipping and cooling, line the sides and base of your chosen tin with baking parchment, making sure there is an overhang on all sides. Place one of the meringue bases on the bottom, chocolate side down. It should fit neatly, with no large gaps around the sides.

When the yolk mixture is cool, remove the bowl from the mixer and use a large rubber spatula to fold in the whipped cream in three batches. Do not over-mix as you want to keep the parfait light and airy, but do make sure that there are no big lumps of cream. Pour the parfait over the meringue base and use a small spatula to smooth and even it out. Carefully place the second meringue sheet over the top of the parfait, chocolate side up. Press gently to ensure the meringue is completely in contact with the parfait. Cover with cling film and place in the freezer for at least 12 hours, or until completely firm.

When ready to serve, remove the frozen parfait from the freezer and, using the overhanging paper, gently pull the whole thing out. If your tin or tray has a removable base, push the parfait up from underneath. Slice into small blocks or rectangles with a warm serrated knife, and serve.