Preheat the oven to 190C/375F/Gas 5.
Place the flour and 50g/2oz butter in a bowl and rub together with your fingertips until the mixture resembles breadcrumbs. Stir in the ground cinnamon and demerara sugar.
Tip the mixture onto a baking tray and bake for 10–12 minutes, or until toasted and golden-brown.
Meanwhile, heat the remaining butter in a lidded frying pan until foaming, then add the pears. Fry for 1–2 minutes, then add the perry and bring to the boil. Reduce the heat until the mixture is simmering, cover and cook for 3–4 minutes, or until the pears are soft.
Spoon the pear mixture into an ovenproof dish and sprinkle over the crumble. Bake for 3–4 minutes.
Meanwhile, to make the cinnamon custard, heat the cream, milk and cinnamon sticks in a saucepan until just boiling, then reduce the heat and simmer gently for 5–6 minutes.
Beat the egg yolks and caster sugar together in a bowl, then gradually add the warm cream mixture, whisking continuously. Return the custard to the saucepan and cook over a low heat for 3–4 minutes, stirring constantly, until the mixture has thickened enough to coat the back of a spoon. Discard the cinnamon sticks.
Serve the pear crumble in bowls with the cinnamon custard.