Directions

• For the genoise sponge
• Heat the oven to 350° Fahrenheit. Line the bottom of two 8″ cake pans with parchment paper and grease generously.
• Melt the butter, then pour into a medium bowl and set aside to cool. Put the eggs, egg yolks, sugar and vanilla into the heatproof bowl of a stand mixer and whisk with a hand wire whisk until combined. Set the bowl over a pan of simmering water (the base of the bowl should not touch the water) and whisk. Using the hand whisk for a couple of minutes, just until the mixture feels warm when you dip in your little finger. and the sugar has dissolved – you shouldn’t feel any gritty grains of sugar. Remove the bowl from the pan and put the bowl in place in the mixer fitted with the whisk attachment. Whisk on high speed until the mixture is thick and mousse-like and it has reached the ribbon stage.
• Whisk the flour and salt into another bowl. Once the egg mixture is at ribbon stage, add the glycerin. Scoop out a cupful and stir it into the cooled butter; set aside. Add a third of the flour mixture on to the rest of the egg mixture and gently fold in. Repeat twice to mix in the rest of the flour, then fold in the butter mixture.
• Divide the batter equally between the prepared pans, and bake for 20 to 25 minutes, until the sponges are well risen and golden, and they spring back slightly when touched.
• Run a knife around the inside of each pan, and leave to cool on a cooling rack.
• For the praline paste
• Reduce the oven temperature to 325° Fahrenheit. Place the hazelnut onto a baking sheet and roast until golden brown, 10 to 15 minutes. Let them cool, then chop roughly and spread onto a baking sheet, lined with parchment paper that was previously greased.
• Put the sugar and water into a medium pan and heat gently, until the sugar has dissolved. Turn up the heat and bring the syrup to the boil, until it turns a rich chestnut-colored caramel. Immediately pour it over the nuts on the baking sheet, covering them.
• Quickly sprinkle the sea salt then leave until cold. Once set, break the praline into shards. Save a few of the best looking shard for the decoration, and grind the rest in a food processor to make a paste.
• For the Swiss buttercream
• Place the egg whites and sugar in the bowl of a stand mixer and whisk until well combined.
• Set the bowl over a pan of simmering water (the base of the bowl should not touch the water) and whisk constantly until the mixture reaches the temperature of 150 degrees Fahrenheit on a sugar thermometer.
• Remove the bowl from the pan and put it in place on the mixer fitted with the whisk attachment. ons.Whisk on high speed to make a thick white meringue. Keep whisking until the meringue has cooled to room temperature, then start adding the butter by the tablespoon, until it’s all incorporated and the buttercream is smooth and silky..
• Fold 6 to 8 tablespoons of praline paste into the buttercream – according to your taste, the rest of the praline will be used for the decorations. Set aside.
• For the vanilla syrup
• Put the sugar, water, and vanilla in a small pan and heat gently until the sugar has dissolved. Bring to the boil then simmer for a minute. Remove from the heat.
• Assemble the cake
• Use a cake board and place one sponge layer on top, crust side down. Brush generously with the vanilla syrup. Leave to soak for 10 minutes.
• Transfer 1/4 cup of the buttercream to a small bowl for the piped decorations, cover and set aside. Spoon about a quarter of the remaining buttercream on top of the layer, and spread evenly.
• Set the other sponge on top of the first layer, crust side up, and brush with the rest of the syrup. Leave for 10 minutes, then use half of the remaining buttercream to evenly coat the top and sides of the cake. Chill until firm.
• Repeat to cover the cake with the rest of the buttercream ( except for that reserved for decoration) – make sure this coating is completely smooth so the mirror-glaze doesn’t have bumps. Now give the cake a thorough chilling in the fridge for about 30 minutes ( or freezer if time is short).
• For the mirror glaze
• Put the gelatin in a microwave safe bowl with two tablespoons of cold water to bloom, for about 5 minutes. Place the chocolate into a heatproof bowl.
• Gently heat the cream, glucose, sugar and water in a medium pan, stirring constantly, until melted and smooth, then bring to the boil. Pour over the chocolate in a thin, steady stream, stirring gently until smooth.
• Warm the gelatin in the microwave until it completely dissolves, then add to
  the chocolate cream mixture and stir in until melted. Blitz with a hand blender to make sure the mixture is completely smooth – take care not
  to introduce froth or bubbles. Leave until cool and thickened but still fluid, stirring frequently.
• Finish assembling the cake
• Remove the cake from the fridge and place on a rack set over a tray to catch the drips. Carefully pour the white chocolate mirror-glaze over the cake to completely and evenly cover the top and sides. Leave to set before transferring to a cake stand.
  To decorate the cake, spoon the reserved buttercream into a piping bag and pipe small ‘dots’ on to the top of the cake in a slightly diagonal line. Add the shards of praline and a little of the praline paste to this line and around the bottom.