First make the pastry. Rub the butter into the flour and salt in a large mixing bowl with your fingertips until the mixture resembles breadcrumbs. Sprinkle over 2–3 tbsp of cold water, then using a round-bladed knife, stir to bind the mixture together. (This can be done in a food processor.) Knead lightly to form a dough, then wrap the pastry in greaseproof paper or cling film and leave to rest in the fridge for 20 minutes.
Meanwhile, put the rhubarb in a saucepan with 3 tbsp of the sugar and cook gently for about 10 minutes, so that it is tender but still holding its shape. Drain well, add the cinnamon and leave to cool.
Preheat the oven to 200°C (gas 6) and put a baking sheet in to heat. Roll the pastry out on a lightly floured surface and use to line a loose-based 24cm fluted tart tin. Prick the base all over with a fork and chill for a further 10 minutes. Cover the pastry with a circle of greaseproof paper and weigh down with dried or baking beans. Place the tin on the baking sheet and bake for 12–15 minutes until the pastry is set and pale golden.
Beat the egg yolk with the remaining sugar, then beat in the crème fraîche or cream.
Remove the beans and greaseproof paper from the pastry case. Pour the crème fraîche mixture into the pastry case, spreading it evenly, then arrange the rhubarb on top.
Return the tart to the oven and bake for a further 15 minutes. Allow to cool for 5 minutes, then carefully remove the tart from the tin. Serve warm.
*You can make the pastry case a day in advance, but do not fill and bake with the rhubarb filling until shortly before serving, or the pastry case will be soggy. *Ceramic baking beans stop a pastry case from rising, when baking ‘blind’. They conduct heat for even cooking and will last for years. However, if you don't have them, simply use dried beans. peas or rice.