Beat the egg yolks with 1/3 cup of granulated sugar until thick and pale. Heat whole milk, remaining sugar, and a pinch of salt until simmering. Gradually temper egg mixture with hot milk, whisking, then transfer back to saucepan. Cook over low heat for about 5-7 minutes until thickened, stirring continuously. Cover with plastic to cool.

Preheat oven to 380°F (190°C) and line two springform pans with parchment paper. In a saucepan, combine water, 1/2 cup of butter, and a pinch of salt. Bring to a simmer, add flour, stirring until a dough forms. Cool for 5 minutes before beating in eggs one at a time until smooth. Spread dough evenly in pans, creating peaks.

Bake for 25-28 minutes, or until puffed and golden with a slight crunch. Keep oven door closed during the first 20 minutes. Cool pastries in pans for 10 minutes before removing and cooling completely on a wire rack.

Cream remaining butter and sugar until light and fluffy. Fold in cooled custard until smooth. Place one layer of cooled choux in a springform pan, spread custard over, and top with second layer of choux. Press down gently.

Chill in refrigerator for at least 1 hour. Before serving, dust with powdered sugar. Slice and serve alongside coffee or tea.