Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper to prevent the puffs from sticking.
In a medium saucepan over medium heat, combine the salted butter, whole milk, water, Dijon mustard, and cayenne pepper. Stir until the butter melts completely and the liquid is hot but not boiling.
Once the butter mixture is hot, remove it from the heat and quickly add the all-purpose flour. Stir vigorously with a wooden spoon until it forms a smooth ball of dough.
Allow the dough to cool for about 5 minutes. Then, add the eggs one at a time, mixing well after each addition until fully incorporated. The dough should be glossy and smooth.
Fold in the grated sharp cheddar cheese and chopped scallions until evenly distributed throughout the dough.
Using a cookie scoop or spoon, drop tablespoon-sized amounts of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Place the baking sheet in the preheated oven and bake for 20-25 minutes, or until the puffs are golden brown and puffed up.
Remove from the oven and let them cool slightly before serving. Enjoy warm for the best flavor and texture!
