Preheat the oven to 180C/Gas Mark 4. Trim the leaves off the rhubarb. Beat the butter, sugar, salt, eggs and yolks together. Add the milk and sift in the flour to make the custard. Butter 4 brûlée dishes or 6 ramekins. Cut the rhubarb into 10cm pieces if using brûlée dishes, or 5cm strips if using ramekins. Line the base of the dishes with the rhubarb strands, laying 5 or 6 lengthways and then another 5 or 6 in an overlapping lattice over the top. Pour the custard over the rhubarb, leaving a ½cm gap at the top (it puffs up as you cook). Repeat with the remaining dishes. Sprinkle the top of each with sugar before popping in the oven for 25 minutes for the brûlée dishes or 20 minutes for the ramekins. The top should be golden and slightly crisp. Leave to cool before serving. You can make one large pudding but allow an extra 5-10 minutes on the cooking time.