For the French toast, place the egg yolks, cream, vanilla seeds, sugar and lemon zest into a large bowl and mix thoroughly. Dip the brioche into the egg mixture.
Heat the butter in a pan until foaming. Add the brioche and fry for 2-3 minutes on each side, or until golden brown. Set brioche aside and keep warm.
For the sauce, melt the butter and sugar together in a pan until golden brown. Add the apple, cinnamon and sultanas to the pan and fry for 3-5 minutes, stirring occasionally to coat the apples.
To serve, place the french toast onto a warmed plate and drizzle with the sauce.