Tip all of the hazelnuts onto a baking tray large enough for them to lay in a single layer, place the tray into a cold oven, then turn the heat on to 180C/350F/gas mark 4. Roast the nuts until the skins have turned dark, they smell toasty and the inside has become a light caramel colour, about 15-25 minutes. Keep a close eye on them as they burn easily.
Once the nuts are toasted, tip them straight from the oven onto a clean tea towel, wrap them up in the towel and rub them together, the skin should flake off, don't worry if you are unable to remove all of the skin. Set aside until cool.
To make the creme patisserie, line a shallow tray with greaseproof paper. Heat the milk and vanilla extract till boiling, meanwhile, whisk the egg yolks and sugar together until thick and pale then sift in the flour and cocoa powder and whisk until smooth.
Gradually pour the hot milk into the egg yolk mixture, whisking the whole time. Pour this mixture through a fine mesh sieve into a clean pan.
Bring up to the boil while whisking then turn the heat down to a simmer and cook for a few more minutes, whisking continuously, until the cream is really thick. Scrape onto the lined tray, cover with clingfilm so that it is touching the surface (to prevent a skin from forming) and place in the fridge until cold.
Make the praline: Line a small tray with greaseproof paper. Put the 75g of hazelnuts into a small, heavy bottomed saucepan with the 75g of caster sugar. Heat while stirring continuously until the sugar has melted and become a caramel colour; quickly tip the mixture onto the lined tray and leave until cool.
Break the cooled praline up into pieces and place in a food processor with the oil. Blitz until it turns into a paste (like peanut butter) then place in the fridge until needed.To make the dacquoise, grind the 125g hazelnuts to a powder, be careful not to over-process them or it may become a paste.Heat the oven to 160°C/325°F/gas mark 3.
Cut out 5 circles of greaseproof paper to line a 15cm/6in round tin (use two or three tins if you have them).Whisk the egg whites with an electric mixer in a large, very clean bowl until soft peaks form, then gradually whisk in the sugar, a little at a time, until the meringue is stiff and glossy.
Carefully fold in the vanilla extract, ground almonds and flour.Gently spread a 5th of the mixture into each of your lined tins, bake for 15 minutes. They will look soft but should not be sticky to the touch.
Remove the layers from the tins, leaving the greaseproof paper on the bottom, and place them on a flat surface (not a wire rack). Quickly wash, dry and re-line your tins, spread more mixture in them and repeat the baking process until you have used all of the mixture and have 5 layers of dacquoise. Leave the layers on the flat surface until cool. You can leave them on the surface, covered with greaseproof paper or a clean tea towel overnight if you wish.
To assemble the torte:
Line your serving tray/cake stand with greaseproof paper.Place the creme patisserie into a bowl and beat with an electric whisk until smooth, beat in the praline paste, divide the patisserie into 5 equal portions.
Remove the paper from the bottom of one of the cake layers and place it on your serving tray. Spread one portion of the creme patisserie over the dacquoise then top with another cake layer. Repeat, making sure that the last layer is placed bottom-side-up (do not place filling on this surface) which will make it easier to obtain a smooth looking finish.Press down gently on the top of the cake to level it out (you want the top to be as flat as possible).Warm the apricot jam until runny then brush a thin layer over the top of the torte, place in the fridge for about half an hour.
Spread the remaining portion of creme patisserie around the sides (not the top) of the torte.
Melt the chocolate and oil together, stir until smooth then place in a piping bag.Mix the icing sugar, oil and lemon juice together, stir in hot water, a teaspoon at a time until the mixture is creamy, but very thick, it shouldn't be runny at all.Spread the icing over the top of the torte using a hot, wet palette knife. Pipe four concentric circles of chocolate on top of the icing then use a skewer to run lines through the chocolate.Press the chopped nuts around the sides of the cake to finish.