- 5 tablespoons unsalted butter, at room temperature, plus more for greasing ramekin
- 1 cup granulated sugar and 1 tablespoon sugar, plus more for dusting ramekin
- Zest of 2 large lemons and juice of 1 of those lemons, strained
- 3 large eggs, separated
- ¼ cup all-purpose flour
- ⅛ teaspoon baking powder
- ¼ teaspoon kosher salt
- 1⅓ cups whole milk
- ⅛ teaspoon cream of tartar (optional)
- Preheat oven to 350 degrees. Fill a kettle and pop it on to boil. Generously grease a single 6- to 8-cup (2-quart) ramekin with butter and lightly dust evenly with granulated sugar. Place prepared ramekin in a large roasting pan.
- Use an electric mixer, with paddle attachment, to cream together butter, sugar and zest on high speed until light and fluffy, about 4 minutes. Then beat in yolks one at a time.
- Add flour, baking powder and salt, and mix until combined. With mixer running at low speed, gradually pour in milk and lemon juice, and mix until smooth. (It’s a runny batter.)
- Fit electric mixer with whisk attachment. In a scrupulously clean, dry mixing bowl, whisk egg whites on high speed for 2 minutes, then add 1 tablespoon sugar and cream of tartar, if using. Continue beating until soft peaks form.
- Use a flexible spatula to gently fold a third of beaten egg whites into batter until incorporated. Add another third, and repeat with remaining third until you have an airy mixture.
- Pour batter into ramekin. Place roasting pan in preheated oven, then carefully pour boiling water into pan until water reaches a couple of inches up the ramekins.
- Bake until golden and puffed, 45-50 minutes. Do not open oven as soufflé bakes. At the table, portion out soufflé into individual bowls. Serve immediately.