Place milk and butter in a saucepan. Heat on medium-low until the butter melts into the milk. Bring to a boil.

Add in 1 cup of flour and reduce the heat. Whisk until a thick, lump-free mass forms that comes off the sides of the pot easily. Remove from stove and cool.

Once cooled, beat in the eggs one by one, mixing until the dough is smooth and a bit sticky. Divide the dough into two.

Grease a large rectangular cake tin (9 x 13 in / 23 x 33 cm) with butter and sprinkle with flour. Spread one part of the dough into the tin and even out the surface with a butter knife or spatula.

Bake at 390?F (200?C) for 25-30 minutes until lightly golden. Do not open the oven during baking. Remove to cool and repeat with the second portion of dough. Leave to cool.

Boil 2 cups of milk with fine sugar and a packet of vanilla sugar.

In a separate bowl, combine 1 cup of cold milk with 1 egg, 4 yolks, potato starch, and all-purpose flour. Mix well.

Stir the milk mixture into the cold milk mixture and bring to a boil, whisking continuously for 3 to 5 minutes until a lump-free custard forms. Remove from heat and cover with cling film to cool completely.

In a mixing bowl, blend 3 sticks of butter and 2 tablespoons of caster sugar until light and fluffy. Gradually add the cooled custard, a few spoonfuls at a time, blending continuously.

Place one layer of baked choux pastry onto a serving plate. Spread the custard cream on top and even out the surface. Cover with the second layer of baked choux pastry.

Chill in the fridge for a few hours before serving. Before serving, sprinkle generously with icing sugar.

Cut into portions with a super-sharp knife.