For the cake, melt the chocolate and butter together, then cool. Meanwhile, heat oven to 150C/130C fan/gas 2 and butter and line the base and sides of a 23cm springform tin with baking paper.
Whisk the sugar and egg yolks until very pale and thick, about 5 mins. Fold in the chocolate mix using a large metal spoon. Put the egg whites and a pinch of salt into another bowl and, with clean beaters, whisk until you have medium peaks. Fold this gently but thoroughly into the chocolate mix with your metal
spoon, then spoon into the tin and bake for 1½ hrs until risen all over. Insert a skewer into the middle of the tin to test; it should come out with just a few damp crumbs but no wet mix. The cake will sink once it cools. Can be frozen up to 1 month ahead.
While the cake cooks, 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. Can be cooked up to a week ahead and kept chilled in some of their poaching liquid.
For the mascarpone layer, half-fill a medium saucepan with water, then bring to a simmer. Put the yolks, sugar and 6 tbsp of the Marsala into a large bowl, sit it over the just-simmering water, then whisk for 5 mins until the mixture is thick and holds a trail for a few secs. 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. Can be made ahead and kept in the fridge. Keep no longer than 2 days in total.
You’re now ready to assemble the trifle. Cut the cake in half – it will be squidgy, so don’t worry if it breaks up. Spoon some of the mascarpone layer into the bottom of a dish, then top with a few pears and a sprinkling of grated chocolate. Put half of the cake on top, then sprinkle with a little of the remaining
Marsala and coffee. Spoon more of the mascarpone over, then top with more pears and more chocolate. Top this with the next piece of cake, spoon over more Marsala and coffee, then spoon the remaining mascarpone mix over the top. Finish with the remaining pears. Chill for at least 2 hrs, or up to 2 days. When ready to serve, scatter with the last of the grated chocolate.