Rub in unsalted butter until mixture resembles breadcrumbs.
Stir in 70ml (5 tbsp) cold water.
Gently knead dough until smooth.
Wrap in clingfilm and place in fridge for half an hour.
Remove dough from fridge and roll it out to line a circular pie dish, or rectangular swiss-roll tin. Thoroughly prick base and return to fridge to chill.
Bake blind until dry and light-golden.
(To bake blind place some greaseproof paper on chilled dough shell and weight it down with some dried pulses or beans, place in a moderately hot oven “ after 15 minutes remove paper and beans and return to oven to dry out for another ten minutes until golden). Reduce oven heat 325, moderately warm oven.
If using pears, peel, quarter them.
Stone and chop the dates in half.
Melt butter in sturdy saucepan with thick base or the dates will burn before they go soft.
Add dates and pears and cook over low heat until soft, at least five minutes.
Spoon fruit mixture into pastry base.
Whisk together the eggs until fluffy.
Add to the beaten eggs, the mascarpone, sugar, and vanilla and beat all together.
Pour over the dates.
The idea is to cover the dates with the creamy egg topping mixture, therefore a very large flat dish would probably not allow the egg mixture to cover the dates adequately; even two smaller pie dishes would be more satisfactory than one over-wide one.
Sprinkle half the brown sugar over the topping.
Bake at 325 until the topping is set (the pastry is pre-baked and the dates and pears are pre-cooked so you only want the topping to turn into a Baked Custard which is why you reduced the heat “ cooking eggs too fast makes them separate, remember watery scrambled eggs and how disgusting it is to see the whey?)
If desired, sprinkle over another 2 spoons brown sugar and flash under a hot grill to caramelise the sugar; this does run the risk of turning your custard watery so a quick flash is plenty.