Step 1

Start by making the custard. Carefully cut the vanilla pod in half lengthways, scrape out the seeds and put in a saucepan with the milk, the empty vanilla pod and ½ of the caster sugar. Warm through over a low heat until the vanilla infuses and the sugar melts, being careful not to scald the milk.

Step 2

In a separate bowl, whisk the egg yolks with the rest of the caster sugar until pale and fluffy, then add the cornflour and mix through. Remove the vanilla pod from the milk, then pour the hot milk, little by little, onto the egg yolks, whisking constantly. This ‘tempers’ the yolks, which gets them used to the heat ahead of cooking.

Step 3

When you have mixed in all the milk, pour the mixture back into the pan and put on a low heat. Slowly warm up the custard while constantly whisking and scraping the bottom of the pan until it thickens enough to really coat the back of a spoon (7-10 minutes). I use a spatula and make sure that no part of it stays on the bottom of the pan for too long, otherwise the egg can overcook and go lumpy. The minute the custard thickens, take off the heat and immediately push it through a sieve into a bowl to cool.

Step 4

If not using the custard straight away, scatter a little icing sugar over the surface to act as a buffer, then press a sheet of baking parchment onto the surface so it doesn’t form a skin in the fridge.

Step 5

Preheat the oven to 220°C, gas mark 7 and put a baking sheet in to heat up. Roll the puff pastry out on the baking parchment it comes wrapped in. Carefully score the edges of the pastry, leaving a 2cm border around the edge. Be careful not to go right through the pastry, but make sure your incisions are deep enough and connect with each other; this lets the edges of the pastry rise.

Step 6

Spread the custard over the inner square of pastry, being careful not to go over the scored borders. Place the rhubarb pieces on top of the custard, nestling them in a little. You want to cram on as much rhubarb as possible. You can make the tart up to this stage, then chill in the fridge for up to 24 hours until ready to bake.

Step 7

Warm the jam in a pan with a tiny splash of water until you have a paintable mixture then, with a pastry brush, paint the rhubarb and the edges of the pastry. This ensures the pastry has a lovely sheen and helps it go golden brown. Carefully transfer the tart onto the hot baking sheet, keeping it on the parchment. Bake for about 20 minutes or until the pastry has picked up some colour and the rhubarb is cooked. Remove from the oven and sprinkle with a little demerara sugar. Leave to cool for a few minutes, but definitely tuck in while still warm.