For poached quince, preheat oven to 130C. Bring sugar and 880ml water to the boil in a casserole, stirring to dissolve sugar. Add quince, vanilla bean and seeds and cinnamon quill. Scatter with reserved trimmings, then bake, covered, until ruby coloured (4-5 hours). Cool in liquid, then reserve one quince for ice-cream. Thinly slice remaining quince and refrigerate in syrup until required.
For quince and sherry ice-cream, blend reserved quince and sherry in a blender until smooth, set aside. Bring milk to the simmer in a saucepan over medium heat. Meanwhile, whisk yolks and sugar in a separate bowl until pale. Pour milk mixture over yolk mixture, stir to combine, return to pan and stir continuously over low heat until mixture thickly coats the back of a wooden spoon (5-10 minutes). Meanwhile, place cream in a large bowl placed over ice. Add custard and quince mixtures and stir occasionally until cool. Churn in an ice-cream machine and freeze until required.
Preheat oven to 170C. Beat butter and 250gm brown sugar until pale and creamy, add eggs, beat to combine. Sift over flour, baking powder and 60gm cocoa powder, stir to combine. Add milk and 60ml sherry, beat until smooth, then stir in chocolate. Pour into a lightly buttered 2.2-litre, 20cm-diameter baking dish and set aside.
Combine remaining brown sugar, cocoa and sherry in a bowl. Add 375ml boiling water, whisk to combine, then pour cocoa mixture over pudding batter. Bake until risen and a skewer inserted into top half withdraws clean (40-50 minutes; cover with foil if browning too quickly). Stand for 5 minutes, then serve warm with poached quince and ice-cream.