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

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

Remove from heat and immediately add all 300g flour at once, stirring vigorously with a wooden spoon until the mixture forms a smooth ball and pulls away from the sides of the pan.

Transfer the dough to a large bowl and let cool for 5 minutes. Add eggs one at a time, beating well after each addition until the dough is smooth and glossy.

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

Bake for 25 minutes until puffed and golden brown, then reduce temperature to 180°C (160°C fan) and bake for another 15-20 minutes until deeply golden and crisp. Do not open the oven door during the first 25 minutes.

Remove from oven and immediately prick the pastry all over with a fork to release steam. Allow to cool completely on wire racks.

For the custard cream: whisk together 750ml milk, sugar, cornflour, and egg yolks in a large saucepan until smooth.

Cook over medium heat, stirring constantly, until the mixture thickens and begins to bubble, about 8-10 minutes. Continue cooking for 2 more minutes while stirring.

Remove from heat and stir in vanilla extract. Cover the surface directly with cling film to prevent a skin forming and cool to room temperature.

Once custard is cooled, beat the softened 150g butter until fluffy, then gradually add the custard, beating until smooth and creamy.

Place one choux layer on a serving board, flat side up. Spread all the custard cream evenly over the top.

Place the second choux layer on top, with the rustic, cracked side facing up to resemble mountain peaks.

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

Dust generously with icing sugar before serving. Cut into squares with a sharp serrated knife.