There's a hidden layer of thick vanilla custard in the centre of
this cake that takes it from good to great - serve it warm, but not
280 gmcaster sugarScrapedseeds of 2 vanilla beans200 gmsoftened butter, plus 10gm extra, melted, for brushing1½ tspground cinnamon2eggs1egg yolk375 gmself-raising flour200 mlbuttermilk 2Granny Smith apples, thinly sliced into roundsThick vanilla custard300 mlmilk60 mldessert wine1vanilla bean split, seeds scraped or 1 tsp vanilla bean paste110 gm(1/2 cup) caster sugar5egg yolks20 gm eachplain flour and cornflourApple-vanilla compote1vanilla bean, split and seeds scraped330 gm(1½ cups) caster sugar75 mldessert wine or brandy5small Granny Smith apples, each cut into 8 thin wedgesFinelygrated rind and juice of ½ lemon
For thick vanilla custard, bring milk, wine and vanilla bean and seeds to the simmer in a saucepan over medium-high heat. Whisk sugar and yolks to combine, then add flours and whisk until smooth. Slowly add hot milk mixture, whisking to combine, then return mixture to pan and bring to the boil, whisking continuously until thick (2-3 minutes). Discard vanilla bean and pour into a buttered 20cm-diameter round cake tin. Smooth top and refrigerate until firm (1-2 hours).
For apple-vanilla compote, stir ingredients and 180ml water in a saucepan over medium-high heat until sugar dissolves, then bring to the boil, reduce heat to medium and simmer until apples are tender and a syrup forms (10-15 minutes). Strain, reserving apples and syrup separately, and refrigerate apples until cool (30 minutes).
Preheat oven to 180C. Rub 20gm sugar and half the vanilla seeds in a bowl with your fingertips to combine, then set aside. Beat butter, remaining sugar, cinnamon and remaining vanilla seeds in an electric mixer until pale and fluffy (4-5 minutes). Scrape down sides of bowl, then add eggs and yolk one at a time, beating well after each addition. Add half the flour, then buttermilk, then remaining flour, stirring between each addition, then spread half the mixture in a buttered 23cm-diameter round cake tin lined with baking paper on the base and smooth top. Unmould vanilla custard and place in the centre, scatter with apple compote, spoon remaining cake batter over, smooth top, then arrange sliced apple in a circle, overlapping the edges. Drizzle with melted butter, scatter with vanilla sugar, then bake until golden and cooked though (1½-1¾ hours; cover with foil partway through cooking if cake starts to brown too quickly and remove foil for the last 10 minutes of cooking). Stand for 15 minutes in tin, then run a small knife around the side and turn out onto a rack to cool slightly (30 minutes). Serve warm, drizzled with reserved syrup.