PREPARE: Heat the oven to 180°C / 350°F / Gas Mark 4. Grease the base and sides of an approximately 26x36cm (10x14in.) high-sided oven tray with butter, then line the base with baking parchment paper.

MAKE THE SPONGE: Using an electric mixer, beat the eggs, sugar and vanilla extract together for 5 minutes until pale and almost tripled in volume. After 5 minutes, drizzle in the oil while still beating. Sift over the flour and baking powder and fold through, keeping as much air in the mixture as possible. Be sure that you have a smooth cake mix with no remaining flour visible.

BAKE THE SPONGE: Pour the mixture into the prepared tray, smooth over the top, and then bake in the preheated oven for 25 minutes until golden brown and springy to the touch. Remove from the oven and immediately run a small, sharp knife around the sides of the cake. Set aside to cool in the baking tray for 15 minutes, then turn out onto a wire rack to cool completely, gently removing the baking paper. (See note)

MAKE THE CUSTARD: Whisk together the cornflour / corn starch and sugar until well combined. Add the eggs and half of the milk and whisk until smooth. 

COOK THE CUSTARD: Heat the remaining milk and alcohol (if using) in a saucepan until nearly boiling. Pour over the cornstarch mixture whisking constantly, then pour the mixture back into the saucepan and cook, stirring until thick. Pour into a large mixing bowl, cover with cling film (directly on the surface) and transfer to the fridge to chill completely (around an hour).

CUT THE CAKE: Cut the cake in half so you have two equal-sized rectangular pieces. Place one rectangle onto a serving dish.

MAKE THE CUSTARD CREAM: Use an electric mixer to whisk the cooled custard until it is light and fluffy. In a separate bowl, beat the cream with a hand-held electric mixer until soft peaks form, then fold it gently but thoroughly through the beaten custard using a large metal spoon or rubber spatula.

ASSEMBLE THE CAKE: Spread two thirds of the custard cream over the base of the cake. Top with the second piece of cake and spread with the remaining custard cream. 

DECORATE THE CAKE: Wash and dry the berries then use them to create a Union Jack decoration on top of the cake. Start with the strawberries to form a cross in the middle of the cake, then use the raspberries to make an ‘X’. Fill the gaps with blueberries, leaving a little space so the white lines of the flag show through.

SERVE AND ENJOY: Chill the cake until needed, then slice and serve.