Preheat the oven to gas 4, 180°C, fan 160°C and butter a large baking dish. Split the buns lengthways and butter their insides. Arrange them butter side up so that they are overlapping each other. They should fit quite snuggly in the dish.
Whisk the milk, eggs and jam together, and blend with the corn flour mix. Pour this mixture over the buttered brioche. The liquid should just be visible at the side of the dish so that plenty of the brioche is still poking out. Scatter with the fresh berries and the demerera sugar and place in the oven for 40 minutes, or until golden brown.
Allow to sit for 5 minutes before serving.
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