1. For crème pâtissière, make a slurry in a bowl with cornflour and 50ml milk. Whisk eggs and sugar in a separate bowl until combined, add flour mixture and stir to combine. Meanwhile, bring remaining milk and vanilla to a simmer in a saucepan over medium-high heat. Pour warm milk onto egg mixture and mix well. Return mixture to pan and stir constantly with a wooden spoon until thick (about 3 minutes), then pour mixture into a clean bowl and whisk to remove any lumps. Whisk in butter in pieces until combined. Cover directly and cool completely, then refrigerate until chilled (2 hours).

2. For brioche, stir milk and yeast and set aside to activate (5 minutes). Mix flour, sugar, salt in an electric mixer fitted with a dough hook, then stir through yeast mixture until a dough forms. While mixing, add eggs one by one, then when incorporated add the butter pieces gradually, incorporating well before adding more, then mix until a smooth dough. Set aside covered to prove until doubled in size (1 hour), then then knock back dough.

3. Roll out dough 2cm thickness and put on a tray into fridge leave for 1 hour.

4. Preheat oven to 180C. Glaze the brioche with eggwash then sprinkle well with rock sugar put back into fridge for 15 minutes. Cut out using a ring mould or an upturned bowl to a neat 22cm round, then bring to room temperature for 15 minutes. Bake until golden and cooked through (30 minutes; if brioche darkens on top while cooking, cover with foil to prevent burning). Cool on a rack.

5. To serve, fold whipped cream into crème pâtissière with the orange blossom water. Halve brioche and brush with Grand Marnier. Spread the cream mixture onto the bottom half and sandwich with remaining half. Sprinkle with icing sugar and serve.