Preparation

• Position a rack in the center of the oven and heat the oven to 400°F. Line a baking sheet with kitchen parchment. In a small saucepan, combine the water, butter, sugar and salt. Bring to a boil over medium-high heat. Add the flour and stir vigorously with a wooden spoon until smooth.
• Remove from the heat and beat with a wooden spoon just until the steam stops rising, about 1 minute. The mixture will smooth out and pull away from the sides of the pan.
• Add one egg and beat well with the wooden spoon. The batter will seem to break apart, but keep working until it becomes smooth.
• Add the second egg and beat again until the mixture is completely smooth.
• Scoop the batter into a pastry bag fitted with a 1/2-inch plain pastry tube (Ateco #5). Hold the bag about 1 inch above a parchment-lined baking sheets and pipe out mounds about 1 inch in diameter. Beat the egg yolk with the milk to make an egg wash. Lightly brush the egg wash on top of each puff with a pastry brush, tapping down any points of dough.
• Bake until the pastries are puffed and deep golden, 22 to 24 min. Let cool on a wire rack. If not using the same day, transfer to a plastic bag and freeze for up to four months; thaw in the refrigerator overnight before using.