For the crepes:

1. Pour flour and salt in a large bowl and make a well in the center. Add the eggs, the melted butter, and the optional sugar if desired. Whisk vigorously starting from the center, and slowly start to pour in a little milk, mixing as you pour, to keep the batter smooth. Pour in the rest of the milk until it looks smooth. Cover the bowl and let sit for 2 hours.**
2. Heat a 15 cm/6-inch crepes pan and grease with oil using a piece of paper towel. Pour in a ladle of dough by tilting the pan in all directions to distribute the dough well. Heat until the surface is cooked, then flip the pancakes and cook the other surface for about a minute, or until the crepe becomes lightly blonde. Proceed the same with the remaining batter.

For the praline spread:

1. Line a baking sheet with parchment paper and set aside.
2. Pour water and sugar in a non-stick pan with a heavy bottom and bring to boil until 245°F (118°C), using a cooking thermometer to measure properly.
3. Lower the heat and add whole hazelnuts and almonds at once, stirring constantly with a wooden spatula. At first, sugar will become white and coat the nuts, and while continuing stirring it will progressively turn into a caramel. This takes about 15-20 minutes.
4. Once the nuts are caramelized, transfer to the prepared baking sheet in en even layer and let cool completely.
5. Gently break down the nut pieces, place into a strong food processor and pulse. At first you will obtain a thin, sandy texture. Continue pulsing until it tuns into a paste, about 15 minutes. Let chill in the refrigerator for about 30 minutes before using.

For the chocolate ganache:

1. In a small saucepan, bring cream, butter and sugar to a boil, and stir until smooth.
2. Remove from heat and pour over the chocolate, chopped in tiny pieces (very important). Let sit for a couple of minutes, then stir until smooth.

To assemble the cake:

1. Place 2 sheets of cling film on a serving plate, forming a cross. Stack 3 crepes in the center and spread with 2 tablespoons of praline spread. Continue to layer a single crepe and 2 tablespoons of filling until you are nearly out of crepes. For stability, occasionally layer two or three crepes instead of one single crepe. End with a layer of 3 crepes.
2. Wrap the cake well in plastic wrap, using a little pressure to keep the cake pressed down. Chill for at least 1 hour.
3. When ready to serve, remove cling film and spread the chocolate ganache on top and add a few extra hazelnuts for topping. Slice in wedges to serve.