Preheat oven to 200°C (180°C fan). Line two 23x33cm baking trays with parchment paper.

For the choux layers: In a large saucepan, combine 250ml water, 250ml milk, 200g butter, and salt. Bring to a boil over medium heat.

Remove from heat and add 250g flour all at once, stirring vigorously with a wooden spoon until the dough forms a ball and pulls away from the sides.

Let the dough cool for 5 minutes, then beat in the 6 eggs one at a time until fully incorporated and the dough is smooth and glossy.

Divide the dough between the two prepared trays, spreading evenly to the edges with a wet spatula.

Bake for 25-30 minutes until golden brown and puffed. Do not open the oven door during baking. Let cool completely.

For the custard: In a large saucepan, heat 750ml milk until steaming but not boiling.

In a bowl, whisk together 200g sugar, 60g cornflour, and 4 egg yolks until smooth.

Slowly pour the hot milk into the egg mixture while whisking constantly, then return everything to the saucepan.

Cook over medium heat, stirring constantly, until the custard thickens and reaches 85°C. Remove from heat immediately.

Add vanilla extract and let cool to lukewarm. Beat in 150g softened butter until silky smooth.

Place one choux layer on a serving platter, spread the custard evenly over it, then top with the second choux layer.

Refrigerate for at least 4 hours or overnight to allow the layers to soften and meld together.

Dust generously with icing sugar before serving, creating a snowy mountain peak effect.