Pre-heat the oven to 180°C / 160°C fan assisted / gas mark 4 / 350°F.
Roughly chop the apples, don’t worry about peeling or coring.
Place the apples and blackberries in a roasting dish and sprinkle over 20g of the sugar. Roast for 1 hour, stirring halfway through, until the apples have softened and the fruit has released its juices. Leave the roasted fruit to cool completely.
Place the cooled roasted fruit into a large, sterilised 1.5 litre preserving jar.
Pour over the vodka, Angostura bitters, pared lemon zest, vanilla extract and cinnamon stick. Stir well to combine, making sure the fruit is fully submerged.
Seal the jar and store in a cool dark place for 2-3 weeks, shaking gently every couple of days. Start tasting from around day 10. The flavour should develop into a balance of roasted apple, blackberry and warming spice. If the cinnamon becomes too dominant, remove it and continue the infusion.
Towards the end of the infusion, make the sugar syrup by gently heating the remaining 80g sugar with 80g water in a small saucepan. Stir until the sugar has completely dissolved, then leave the syrup to cool completely.
Strain the infused vodka through a fine sieve lined with muslin into a clean jug. Allow the liquid to drain naturally rather than pressing the fruit too firmly, which can make the vodka cloudy.
Add cooled sugar syrup gradually, tasting as you go, until the vodka reaches your preferred level of sweetness. You may choose not to use all of the syrup. Pour into a final sterilised airtight bottle and seal.
Leave the bottled vodka to rest for at least 2 weeks before serving. The flavours will soften and become more rounded over time.