1. Lightly beat the eggs in large mixing bowl. Sift in the flour and salt and stir together thoroughly. Add 2 tbsp of the warm milk and beat until the mixture is completely smooth and free of any lumps. Slowly beat in the remaining milk and rum and leave the batter to stand for a good 3 hours.
2. Preheat the oven to 220C/425F/gas mark 7.
3. Peel, halve, core and thinly slice the pears. Grease a 20-23cm loose-bottomed cake tin with butter and put in a preheated oven briefly to heat it. Remove from the oven and immediately pour in the batter and arrange the pear slices on top, dot with butter and return to the oven. Bake for 15 minutes, then lower the temperature to 200C/440F/gas mark 6 and bake for a further 20-25 minutes until well puffed up and golden brown. If, towards the end of cooking, the top looks in danger of catching, cover loosely with a sheet of greaseproof paper.
4. Transfer to a serving plate, sprinkle with the caster sugar and serve immediately while still hot.
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