How to Make It
Zest 2 of the lemons and set zest aside. Juice all lemons until you have 1/4 cup lemon juice; set aside.
Whisk together sugar, flour, lemon juice, and egg yolks until homogenous. In a saucepan, heat milk until just beginning to steam but not boiling. Remove from heat and slowly stream in half of the warm milk into the egg mixture while whisking. Once fully combined, return saucepan to medium heat and whisk milk while streaming in the warm egg mixture. Continue whisking until mixture thickens, about 5 minutes. Remove from heat; add vanilla and 1 tablespoon butter; stir until melted and combined. Set aside to cool, about 10 minutes.
While custard is cooling, rub the inside of four 7-ounce ramekins with 1/4 teaspoon butter per ramekin. Divide 1 tablespoon sugar between 4 ramekins and turn each ramekin around so the sugar coats the entire inside surface. Discard excess sugar inside ramekin. Place ramekins on a baking sheet.
Preheat oven to 350°F. Place egg whites in the bowl of a mixer and beat on medium-high with whisk attachment until whites begin to foam, 1 to 2 minutes. Slowly stream in remaining 2 tablespoons sugar and beat until soft peaks form, 2 to 3 minutes.
Transfer cooled custard to a mixing bowl. Using a rubber spatula, fold half of the egg whites into the custard until no streak of white remain. Add remaining egg whites and gently fold until fully combined, being careful not to stir the mixture. Divide batter between prepared ramekins.
Bake until tops are golden brown, 28 to 30 minutes. Dust with powdered sugar and serve warm.