First, make the crème pâtissière. Put the blood orange zest and juice in a large saucepan, scrape in the beans from the vanilla pod and bring to the boil over a medium-high heat. In a large bowl, whisk together the whole eggs, yolks, sugar and cornflour for 2-3 mins until smooth. Pour the orange juice mixture over the egg mixture while whisking together to combine. Pour back into the pan and simmer, whisking constantly, for 4-5 mins until the custard has thickened. Scrape the custard into a clean bowl and cover closely with cling film, pressing it onto the surface to prevent a skin from forming. Allow to cool to room temperature, then put in the fridge for at least 3 hrs to chill completely.
To make the crêpe batter, put the flour, sugar and 1 tsp salt in a large bowl and mix together. In a separate bowl, whisk together the eggs, three-quarters of the milk and the butter to combine. Make a well in the flour mixture and pour in the egg mixture. Whisk together into a smooth batter. Cover with cling film and leave at room temperature for 30 mins before cooking. If the batter is very thick, gradually loosen with the remaining milk until it’s the consistency of double cream.
To cook the crêpes, place a 20-25cm non-stick crêpe or frying pan over a medium heat and lightly grease with vegetable oil. Ladle a thin layer of the batter into the pan, swirl around so it’s even, then tip any excess batter back into the bowl. Cook for about 11/2 mins or until lightly browned underneath, then flip and cook for a further 30-40 secs until lightly browned. Tip onto a plate lined with baking parchment. Repeat with the remaining batter and pile the crêpes on top of one another, using pieces of parchment to separate them. You should have about 20-25.
To finish the crème pâtissière, remove the custard from the fridge and beat until loosened and smooth. In a separate bowl, whisk the cream until it holds medium peaks. Fold the cream into the custard in 3 additions until fully combined. To assemble, place the first crêpe on a serving plate or cake stand and top with a small amount (about 3 tbsp) of the crème pâtissière. Use a palette knife or spoon to spread into a thin, even layer that almost reaches the edge. Repeat with the remaining crêpes and filling until it has all been used up. Cover the cake lightly with cling film and chill until ready to serve.
To make the caramelised orange slices, put the sugar in a big pan with 400ml water and bring to the boil. Add the orange slices and boil gently for about 20 mins or until the orange peel is starting to turn translucent. Reduce the heat to a gentle simmer and cook for 10-20 mins more or until the liquid has reduced to a thick syrup. Carefully remove the orange slices from the pan and place on a baking tray lined with parchment, and leave to cool completely. When ready to serve, top the cake with the slices. Best served on the day it’s made, but will keep in the fridge for up to two days. Any leftover syrup can be used in cocktails.