Begin by melting 80 grams of butter in a saucepan over low heat. Stir in 100 ml of milk, 100 ml of water, and ½ teaspoon of salt until combined. In a separate bowl, whisk together 500 grams of flour and 1 teaspoon of baking powder. Gradually pour the melted mixture into the dry ingredients, stirring until the dough forms and pulls away from the sides of the pan, approximately 5 minutes.

Transfer the dough to a mixing bowl and let it cool for about 10 minutes. Once slightly cooled, add 4 eggs one at a time, mixing thoroughly after each addition until the dough is smooth and glossy.

Preheat your oven to 400°F (200°C). Divide the dough into two equal portions and spread each portion evenly in a greased rectangular baking pan. Bake for around 20 minutes or until the tops are golden brown.

In a saucepan, beat 4 egg yolks with 150 grams of sugar until frothy. Stir in 1 tablespoon of flour and gradually add 100 ml of milk, cooking on medium heat until thickened, roughly 5-7 minutes.

Once the cream filling has cooled, beat 150 grams of softened butter in a mixing bowl until fluffy. Gradually add the cooled cream mixture, mixing well until smooth.

To assemble, place one baked layer on a serving platter, spread half of the cream filling, place the second layer on top, and spread remaining filling. Refrigerate for at least two hours.

Before serving, dust the top with icing sugar and slice into pieces.