Preheat oven to 200°C (180°C fan). Line two large baking trays with parchment paper and draw a 25x35cm rectangle on each.

For the choux layers: combine water, milk, butter, and salt in a large saucepan and bring to a rolling boil over medium-high heat.

Remove from heat and add all the flour at once, stirring vigorously with a wooden spoon until the mixture forms a smooth ball that pulls away from the sides.

Let the dough cool for 5 minutes, then beat in eggs one at a time, mixing thoroughly after each addition until smooth and glossy. Stir in vanilla extract.

Divide the choux paste between the two prepared trays, spreading evenly within the marked rectangles to create rough, mountainous surfaces.

Bake for 25-30 minutes until puffed, golden brown, and crisp. Do not open the oven door during the first 20 minutes. Let cool completely on wire racks.

For the custard cream: whisk egg yolks, sugar, and cornflour together in a bowl until pale and smooth.

Heat 500ml milk in a saucepan until just simmering, then slowly pour into the egg mixture while whisking constantly.

Return mixture to the saucepan and cook over medium heat, whisking continuously until thickened, about 5-7 minutes. Remove from heat and stir in vanilla.

Cover custard with cling film touching the surface to prevent a skin forming. Refrigerate until completely cold, about 1 hour.

Beat the softened butter until fluffy, then gradually fold into the cold custard until smooth and creamy.

Place one choux layer on a serving plate, rough side down. Spread the custard cream evenly over the top.

Place the second choux layer on top, rough mountainous side facing up. Refrigerate for at least 2 hours to set.

Dust generously with icing sugar before serving. Slice with a sharp serrated knife.