Preheat oven to 180/160C fan. Line and generously butter the base and sides of the cake tin.
Wash the pears and leave the skins on, then use a mandoline or sharp knife to slice them lengthways, 2 millimetres thick.
Put the 150 grams caster sugar and dark brown sugar and water into a saucepan over a low heat and stir until the sugar has dissolved. Working in batches, add the pear slices and let them simmer for a few minutes until pliable but not mushy. Arrange pear slices around the bottom of the tin, with the small ends meeting in the centre, and the slices slightly overlapping each other. Carry on layering the pears across the edges of the tin and up the sides, leaving a 2-3 centimetre overhang.
Chop the chocolate very finely. Tip into a mixing bowl and add the almonds, remaining 200 grams caster sugar, salt, milk and then the egg yolks one at a time. Mix well. Whisk the egg whites to firm peaks then mix ¼ of them through the almond mixture to loosen it up, before gently folding the rest in. Once just mixed, tip the mixture into the prepared tin. Fold the overhanging pear slices over the top of the batter then bake for 60 minutes, until a skewer comes out clean. Leave to cool slightly then tip the cake out upside down onto a serving plate, and remove the baking paper. Serve warm or leave to cool.