To make the crème pâtissière, pour the milk into a pan. Split vanilla pod down its length using a small sharp knife (do not cut in half completely). Twist tip of the knife inside the pod to open out like a book at one end, then run the knife down its length to scrape out seeds. (See step 1)
Put the seeds and empty pod in a medium-sized pan with the milk and bring slowly to a gentle boil. Reduce the heat and simmer for a couple of mins. (Step 2)
Put egg yolks, sugar and corn flour into a bowl. Whisk with a hand electric mixer until the mixture becomes pale and fluffy – it should leave a trail that stays on the surface momentarily when the whisk blades are lifted. This whisking will take about 2 mins. (Step 3)
Pour about a third of the hot milk into the bowl, whisking on a slow speed all the time until it has all been mixed in. Now whisk in the remaining milk, removing pod. (Step 4)
Return the mixture to the pan, scraping it out of the bowl using a rubber spatula. Bring slowly to the boil, stirring all the time, until the custard is thickened, smooth and glossy. At first, it will look a bit lumpy, but persevere – keep stirring and it will become smooth. Reduce the heat and simmer for 2 mins, stirring. (Step 5)
Remove the crème pâtissière from the heat. Cover the surface of the crème pâtissière directly with cling film (this will stop a skin forming). Leave to cool, then chill in the fridge until needed. (Step 6)
Heat oven to 190C/170C fan/gas 5. Divide the pastry into 8 equal pieces. Roll out each one to line a 7-8cm deep fluted tartlet tin, trim leaving a little pastry sticking up above the tin. Line each one with a paper cake case and add a layer of baking beans. Put on a baking tray and bake for 10 mins, then remove the paper and beans and bake for a further 5 mins until the pastry is crisp and golden.
Trim any pastry edges that need it with a small sharp knife. Remove from the tins and leave to cool on a wire rack.
Halve or quarter any large strawberries and remove the stalks from any redcurrants, then mix the fruits together. Fill the pastry cases with crème pâtissière and pile a generous layer of fruits on top. Dust thickly with icing sugar and serve.