Preheat the oven to 350°F (175°C).
In a medium saucepan, combine the milk, butter, and salt with 1/2 cup of water and bring to a boil. Add the sifted flour and stir with a wooden spoon until thoroughly combined into a dough. Continue stirring over medium heat for about 3 minutes.
Transfer the dough into the bowl of a standing mixer. Stir at low speed for a few minutes to lower the temperature of the dough. Increase the speed to medium and then add the eggs, one at a time. Mix until a smooth, cool dough forms, about 4 minutes.
Fill a pastry bag, fitted with a #9 tip, with the dough, or use a soup spoon to form small puffs, about 2 inches in diameter, on a parchment-lined baking sheet. Brush the puffs with the beaten egg yolk and transfer to the oven.
Bake for 45 minutes, or until golden brown. Cool the puffs on a wire rack and then slice them, as if they were hamburger buns, with a serrated knife.