For the nut praline (can be prepared up to three days ahead):
For the ganache (make at least 4 hours ahead):
100gdark chocolateideally 60% cocoa chopped
100gmilk chocolateideally 40% cocoa chopped
For the chestnut-chocolate cake:
170gdark chocolateideally 60-70% cocoa
To prepare the nut praline:
Preheat the oven to 300°F. Scatter nuts on a baking sheet lined with parchment paper, or a silicone baking mat, and roast for 10 minutes.
Meanwhile, pour the 80g sugar into a heavy-bottomed saucepan with 3 tablespoons of water and bring to a boil over medium heat to melt the sugar. Add the butter and pecans while the nuts are still warm. The sugar will be slightly grainy at first, but as you continue to stir and cook, the sugar will melt and form a golden caramel coating over the nuts (this will take about 10 minutes).
Pour the caramel and nuts onto a parchment-lined baking sheet and let cool. Break into shards and keep in a box away from moisture at room temperature for up to three days.
To prepare the ganache:
Prepare at least four hours in advance, because the ganache needs to be completely chilled before it can be whipped into an icing-like consistency. To begin, place the chopped dark and milk chocolates into a large, heat-proof bowl and set aside. Then, pour the 100g sugar into a medium-sized heavy-bottomed saucepan. Melt the sugar over medium heat, without stirring, until it turns a golden brown colour.
Heat 200ml of the cream in a separate saucepan.
When the sugar has caramelized, add the butter, stir, and then pour in the hot cream (be careful of hot sputtering sugar).
Stir until the sugar and cream meld together into a smooth liquid, then pour onto the chopped chocolate. Stir the cream/caramel/chocolate mixture gently until it's well blended, add the remaining 100ml cream and mix well.
Place this in the fridge to cool for about four hours (or 30 minutes in the freezer if you're short on time).
To prepare the cake:
Preheat oven to 350°F. Butter your cake pan, line bottom with parchment paper (I used an 8" round cake pan) and wrap the bottom of the pan with tin foil and set on a baking sheet.
Whisk the eggs and sugar together in a medium-sized bowl until light and fluffy.
Meanwhile, melt the chocolate in a double-boiler over very low heat (or in the microwave).
When eggs and sugar are whisked, add the melted chocolate in two batches, stirring gently between each addition, then add the sifted chestnut flour and stir gently until just combined.
Carefully pour the cake batter into your prepared pan, and bake for 15 to 20 minutes (check at 15 minutes - mine was well done at 18 minutes). The cake should be just pulling away from the edges of the pan, and maybe cracking a bit on top, but still slightly moist in the centre, somewhat like a brownie.
To assemble the cake:
When the cake has completely cooled, it's ready to be iced. Take the cold ganache out of the fridge, and whip with either a hand or standing mixer at medium speed until it becomes lighter and mousse-like. Be careful to not over whip, as it can become grainy.
Place the ganache in a piping bag and pipe ganache in whatever design you like over the cake. Garnish with the nut praline fragments.