Preheat the oven to 180C/350F/Gas 4.
First make the pastry. Cream the butter and sugar together by hand or in a food mixer (no need to over cream).
Add the eggs and beat for several minutes. Reduce speed and mix in the flour.
Turn out onto a piece of floured greaseproof paper, flatten into a round wrap and chill. This pastry needs to be chilled for at least 1 hour otherwise it is difficult to handle.
To make the tart, roll out the pastry 3mm/1/8in thick approximately and use about 2/3rds of it to line a 21.5cm/8¾in square x 2.5cm/1in deep with fluted edge tin.
Lay the rhubarb in the pastry base and sprinkle with the sugar.
Cover with a lid of pastry, seal edges, decorate with pastry leaves, brush with the egg wash and bake in the preheated oven until the rhubarb is tender, approximately 45 minutes to 1 hour (but do keep checking some ovens may vary cooking time).
When cooked, cut into squares, sprinkle lightly with caster sugar and serve with softly whipped cream and Barbados sugar.
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James Martin is joined by top chefs Mark Sargeant and Ben Tish and wine expert Olly Smith.