A grown-up cake for a stylish celebration, this cake is moist, richly flavoured with coffee and has lovely praline-filled layers, too.
Heat the oven to 180°C/160°C fan/Gas 4.
Beat the butter and sugar together in a stand mixer fitted with the beater, on medium speed for about 5 minutes, until pale and fluffy. Meanwhile warm 125ml of the milk in a small pan with the instant coffee, then remove from the heat and add the cold milk.
Add the beaten eggs to the creamed mixture, little by little, mixing well between each addition. Add half the flour, all the baking powder and half the milk mixture. Beat for 30 seconds, then add the remaining flour and milk mixture and beat for 1 minute until smooth.
Divide the mixture proportionately between all the tins. Bake the smallest sponges for 25 minutes, the middle-sized sponges for 30 minutes and the largest sponges for 40 minutes, until each feels just firm to the touch and a skewer inserted into the centre comes out clean. Leave to cool in the tins for 5 minutes, then turn out onto wire racks to cool completely.
Meanwhile, make the coffee syrup. Place the coffee liqueur, coffee and sugar along with 75ml of water in a small pan over a medium heat. Bring to the boil and boil for 3–4 minutes, or until slightly syrupy. Leave to cool.
Make the praline. Spread the hazelnuts on the foil-lined baking tray. Place the sugar in a medium pan over a medium heat and cook, shaking gently (but don’t stir) until all the sugar has dissolved. Increase the heat to high and allow the sugar to caramelise to a golden brown. Remove from the heat immediately and pour the syrup over the hazelnuts. Add the pinch of flaked sea salt and leave the praline for about 15 minutes to set. Once set, break it up, then place it in the food processor to blitz to a coarse, slightly chunky powder. Set aside.
Make the Italian meringue buttercream. Place the sugar and 60ml of water in a small pan over a low heat. Cook without stirring until the sugar has dissolved, then increase the heat and cook until the syrup reaches 115°C on a sugar thermometer.
While the sugar is dissolving, whisk the egg whites in the clean bowl of a stand mixer fitted with the whisk until they form soft peaks. With the whisk running, pour the syrup into the egg whites and keep whisking until the bowl is cool.
Add the butter a few dice at a time, until it is completely combined. Stir in the cooled espresso and the coffee extract. If necessary, chill the icing until it is thick enough to spread.
Make the chocolate triangles. Melt each chocolate in a separate small, heatproof bowl over a pan of gently simmering water. Spread each thinly onto a separate sheet of baking paper, leave to set in the fridge. Then, using a hot knife, cut out different triangle shapes and leave to set in the fridge again.
For the pearls, roll the sugar paste into large pea-shaped balls, then roll in gold leaf to coat.
To assemble, slice the 20cm cakes in half horizontally and brush each cut surface with the syrup. Spread a thin layer of buttercream onto one half of each cake. Sprinkle a thin layer of hazelnut praline on top, then sandwich the cakes with their top halves. Spread another layer of buttercream and praline on top of one of the cakes and stack the other cake on top. Place on the serving plate and very thinly spread the top and sides of the stacked cake with buttercream. Chill for 15 minutes.
Repeat with the pairs of smaller cakes, placing each stacked cake on the appropriate cake boards and spreading all over with buttercream.
Cut the cake dowels to the height of the largest cake, then place them in a triangle formation in the middle of the cake, pushing down all the way through to the serving plate. Stack the smaller cakes on top.
Arrange the chocolate triangles around the base of the cake, with a few tall triangles on top and place the sugar-paste pearls randomly around the cake to serve.