Preheat the oven to 350 degrees and line two baking sheets with parchment paper (I used a silicone baking mat); set aside.
In a large bowl, using a spatula, cream together 1/2 cup (1 stick) unsalted butter - room temperature, 1 cup packed brown sugar, and 1/4 teaspoon salt. I didn't have quite enough dark brown sugar so I had to add a wee bit of light brown to fill the cup. It'll be okay and dark brown sugar goes onto the grocery list. The mixture will become almost pasty.
Next, add 1 egg, 1 teaspoon vanilla, and 1/4 cup water; mix until well combined.