1. Bring butter, sugar, salt, and 1 cup water to a boil in a medium saucepan. Remove from heat. Using a wooden spoon, quickly stir in flour. Cook over medium-high heat, stirring constantly, until mixture pulls away from sides and a film forms on bottom of pan, about 3 minutes.

2. Transfer to the bowl of an electric mixer fitted with the paddle attachment. Mix on low speed until slightly cooled, about 1 minute. Raise speed to medium; add whole eggs, 1 at a time, until a soft peak forms when batter is touched with your finger. If peak does not form, lightly beat remaining egg white, and mix it into batter a little at a time until it does.