Preheat oven to 350°F with rack in middle. Butter a 1 1/2-quart shallow baking dish.
Whisk together flour and salt in a large bowl.
Whisk together milk, lemon juice, Scotch (if using), butter, honey, zest, and yolks, then stir into flour mixture.
Beat whites with an electric mixer at medium speed until frothy. Increase speed to medium-high and add sugar, 1 tablespoon at a time, beating until whites just hold stiff peaks.
Stir about one third of whites into flour mixture to lighten, then fold in remaining whites gently but thoroughly.
Pour batter into baking dish and bake in a water bath until puffed and golden-brown, 45 to 50 minutes.
