For the base; mix the flour, Baking Powder and 200g of the sugar, rub in the butter with your fingertips until it resembles fine breadcrumbs. Beat one of the eggs with 1tbsp of water then mix in using a butter knife. Then, without overworking, bring the mixture together with your hands to make a smooth dough. Cover and leave in the fridge for 30 mins.
For the filling; in a wide pan bring 90ml of water mixed with 50g of sugar to the boil, reduce to a simmer then add the rhubarb in a single layer (you may need to do two batches), cover and cook for 3-4 mins until just tender. Remove with a slotted spoon and set aside on a plate to cool. In a jug mix the rest of the sugar, the eggs and yolk, cream and Vanilla Extract until just combined, set aside.
Heat the oven to 180C, 160C fan, gas 4. Grease a 20cm x 30cm tray or brownie tin. Take a sheet of baking parchment big enough to line the whole tin, trace the base of the tin in the middle then roll the dough out to cover it. Trim and reserve the excess pieces of dough. Snip from the tip of the parchment to the tip of the dough in each corner then set into the tin, tidy up the parchment and gently press the dough to the edges. Bake for 20 mins.
Turn the oven down to 150C, 130C fan, gas 2. Carefully press strips of the reserved dough all along the edges of the tin to seal in the dough after any shrinkage from baking. Scatter the rhubarb on top of the dough and pour over the custard. Bake for 40 mins or until the centre is set but with a slight wobble. Cool for at least 30 mins.