Pre-heat the oven to 150°C and line and grease a 20cm round cake tin.
Melt the chocolate in a bowl over a pan of simmering water or a bain marie then leave to cool slightly whilst you get on with the rest.
Whisk up the egg whites in a food mixer or by hand until they hold a stiff peak and set aside.
Beat the egg yolks, sugar and vanilla extract until pale and thick.
Pour the melted chocolate into the egg and sugar mixture and beat in to combine thoroughly.
Add the ground hazelnuts, cocoa powder, baking powder and espresso powder and mix well. Stir in the whole milk to slacken the mixture.
With a metal spoon add a third of the egg whites into the batter gently folding it in until just combined then fold in the remaining whites.
Pour the batter into the prepared cake tin and bake for 10 minutes then turn the oven up to 160°C and continue baking for around 35-40 minutes until firm to the touch and an inserted toothpick comes out clean.
Once the cake is ready, remove from the oven and leave in the tin whilst you prepare the chocolate soaking syrup.
The Chocolate Soaking Syrup
For the chocolate soaking syrup heat up the water and sugar in a small saucepan until the sugar has melted and it has reached a gentle boil. Whisk in the cocoa to dissolve then remove from heat and stir in the vanilla.
Poke holes in the surface of the cake with a cocktail stick, pour the syrup evenly over and let the cake cool in the cake tin before removing.
The Hazelnut Praline
To make the hazelnut praline heat the caster sugar in a medium sized saucepan, without stirring, until it has all melted and is turning a golden brown. Add the sea salt then very quickly pour in the hazelnuts, turning into the melted sugar, then pour out onto baking parchment.
Leave the praline for about 5 minutes until just cool enough to handle then pull the nuts apart so that the caramelised sugar is pulled into strands. Then roughly chop some of the hazelnuts. Leave to cool whilst you prepare the chocolate glaze.
The Chocolate Glaze
To make the glaze mix all the ingredients in a saucepan and heat until it has all melted together and smooth.
Remove from the heat then pour over the cake, allowing some of the glaze to drip down the sides.
Sprinkle the whole nut praline and the chopped nuts onto the top of the cake. Leave for the glaze to set then serve.