1/4 cup all-purpose flour
1/4 teaspoon salt
1 cup whole milk
1/3 cup fresh lemon juice
1/4 cup Scotch (optional)
1/2 stick unsalted butter, melted and cooled
3 tablespoons mild honey
1 teaspoon grated lemon zest
3 large eggs, separated, at room temperature 30 minutes
1/3 cup sugar
Preheat oven to 350?F with rack in middle. Butter a 1 1/2-qt 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 Tbsp 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 (see Tips) until puffed and golden-brown, 45 to 50 minutes. Serve warm.