Start by preheating the oven to 250°C (480°F) with fan assisted.
Grease the molds and dust them with unsweetened cocoa powder.
Melt the chocolates with the milk, heavy cream, salt, and butter over a double boiler on low heat.
Beat the eggs and sugar together until the mixture becomes pale and slightly thickened, then add the flour. Mix well.
Combine the two mixtures and add toasted chopped hazelnuts if desired (I used roasted diced almond).
Pour into moulds.
Bake on the middle rack for 4 to 5 minutes, depending on the size of the molds.
The chocolate molten cakes should be firm on the outside to be able to unmold them.
Enjoy immediately, dusted with powdered sugar. You can accompany this chocolate molten lava cakes with a scoop of vanilla ice cream.