Preheat the oven to 200ºC, fan 180ºC, gas mark 6; place a baking sheet in the top half of the oven to heat up. For the pastry, put the ﬂ our and butter in a food processor and pulse until it looks like ﬁne breadcrumbs (or rub between your ﬁngertips to a crumb texture). Add the Golden Caster Sugar and pulse a couple of times to distribute, then add the egg yolk and 2 tbsp ice-cold water. Pulse until the pastry just comes together to form a dough. Do not over process.
Turn the dough onto a lightly ﬂoured surface (no need to wash the food processor as you will re-use it for crumble). Roll out the dough to form a circle that is large enough to cover the base and sides of the tart case with some overhang. Roll the pastry over your pin and gently lay over the case. Gently press into the base and up the sides, allowing the excess to fall over the edges. Roll the pin over the top to cut away the excess, prick all over the base with a fork, then pop in the freezer for 15 minutes or the fridge for 30 minutes.
When ready to bake, line with parchment and baking beads. Bake for 10-15 minutes until sides are just turning golden. Remove the parchment and beans and bake for another 10-15 minutes until light golden all over. Lightly beat the egg white and, as soon as the case comes out of the oven, brush over the base and sides to help seal the pastry and prevent it going soggy when you add the ﬁlling. Cool completely.
For the ﬁlling, in a large-lidded sauté or frying pan, heat the butter until fairly hot and starting to brown. Toss in the apple and fry for 30 seconds or until starting to brown. Toss 100g of the Golden Caster Sugar together with the apple over a medium-high heat, then reduce to low and add 2 tbsp water. Cover with a lid and gently steam for about 3-4 mins until just soft. Take off the heat and stir in the blackberries, leave covered for a few more minutes. Check for sweetness, adding a little extra sugar if needed. Uncover to cool completely.
For the crumble, place the ﬂ our and butter in the food processor and pulse to ﬁ ne breadcrumbs. Pulse in the Light Brown Muscovado Sugar once or twice, then tip into a large bowl and work with ﬁngertips to make larger crumbs.
Strain the fruit ﬁlling over a bowl and catch the juices. Spoon the fruit into the bake, ﬁlling to 3/4 of the way up. (Keep any leftover and puree with the juices to serve as a coulis over the pudding). Sprinkle an even layer of crumble over the fruit, then bake for 25-30 minutes golden. Cool for 5 minutes before serving.
Meanwhile make the custard. In a large bowl, whisk together the Golden Caster Sugar, cornﬂour, egg yolks and Vanilla Bean Paste until smooth. Place the milk and cream in a large heavy-based saucepan and bring to the boil. Take off the heat and slowly pour through a ﬁ ne sieve over the egg yolk mix, whisking constantly to combine.
Rinse out the pan, then pour the custard back in. Gently heat, stirring constantly for about 5-8 minutes, or until thickened. Serve with the tart.