Add your chilled butter, sugar, egg yolk, flour, and salt into a food processor. Pulse until the mixture looks crumbly, similar to coarse sand.

Pour in the water a little at a time, pulsing after each addition. Stop as soon as the dough starts clumping together into a ball.

Transfer the dough onto a floured surface and knead gently until it feels slightly tacky but not sticky. Divide into two portions and wrap each in plastic wrap. Refrigerate for 1 to 2 hours.

Preheat your oven to 375°F (190°C) and wrap metal clothespins or similar molds tightly in foil.

On a floured surface, roll one dough portion into a 12x8 inch rectangle, slice into 1-inch strips.

Wrap each strip around a prepared mold, overlapping slightly, and place seam-side down onto a parchment-lined baking sheet.

Bake for 18 to 20 minutes, watching for a light golden color. Start checking around the 16-minute mark.

Let the shells cool for 2 to 3 minutes, then carefully twist and slide the molds out. Allow the empty shells to cool completely before filling.

In a saucepan over medium heat, whisk together the flour, sugar, and milk. Keep whisking for about 10 minutes until it thickens.

Transfer the thickened mixture into a bowl, press plastic wrap onto its surface, and refrigerate for about 40 minutes.

Beat the softened butter alone for about 8 minutes until pale and fluffy.

Gradually add the cooled pudding mixture into the whipped butter while mixing. Add the vanilla extract and salt, continuing to beat for another 10 to 13 minutes until smooth.

Transfer the cream into a piping bag and pipe generously into each cooled shell from both ends.