Melt the chocolate and butter together, then cool.
Meanwhile, preheat the oven to 150C/130C fan/gas 2 and butter and line the base and sides of a 23cm springform tin with baking paper.
Step 2: Cake Batter
Whisk the sugar and egg yolks until very pale and thick, about 5 mins.
Fold in the cooled chocolate mix using a large metal spoon.
In another bowl, whisk the egg whites and a pinch of salt until you have medium peaks.
Gently but thoroughly fold the egg whites into the chocolate mix with a metal spoon.
Spoon the batter into the prepared tin and bake for 1½ hrs until risen all over.
Check for doneness with a skewer – it should come out with just a few damp crumbs but no wet mix.
The cake will sink once it cools.
It can be frozen up to 1 month ahead.
Step 3: Poached Pears
While the cake bakes, put the pears, vanilla pod, and 1 litre water into a saucepan.
Weigh the pears down under the surface with a small plate, then simmer for 20 mins, covered, until tender.
Leave to cool in the liquid if you have time.
Cut each pear into 6 long slices, then remove the stalk and the core.
This can be done up to a week ahead, and the pears can be kept chilled in some of their poaching liquid.
Step 4: Mascarpone Layer
Half-fill a medium saucepan with water and bring to a simmer.
In a large bowl, whisk the yolks, sugar, and 6 tbsp of Marsala over the simmering water for 5 mins until the mixture is thick and holds a trail for a few seconds.
Put the mascarpone into a bowl, beat with 2 tbsp more Marsala to loosen, then whisk in the egg mix in 2 batches until smooth, thick, and light.
This layer can be made ahead and kept in the fridge, but not longer than 2 days in total.
Step 5: Assembly
Cut the cake in half and spoon some of the mascarpone layer into the bottom of a dish.
Top with a few poached pears and a sprinkling of grated chocolate.
Place half of the cake on top, sprinkle with a little of the remaining Marsala and coffee.
Spoon more mascarpone over the cake, then top with more pears and chocolate.
Add the second piece of cake, sprinkle with more Marsala and coffee, then spoon the remaining mascarpone mix over the top.
Finish with the remaining poached pears.
Chill for at least 2 hours, or up to 2 days.
When ready to serve, scatter with the last of the grated chocolate.
