Heat the oven to 325°F. Line the bottom of a roasting pan or 9x13-inch baking pan with a small kitchen towel (fold as needed so it lies completely flat). Lightly coat 8 (6-ounce ramekins) or an 8-inch square baking pan with cooking spray and place in the roasting pan. Bring about 7 cups water to a boil in a kettle or saucepan, then turn off the heat.
Whisk 4 large egg yolks, 1 cup of the granulated sugar, ¾ cup whole milk, the finely grated zest of ½ medium lemon, ½ cup lemon juice, 2 tablespoons melted unsalted butter, ½ teaspoon vanilla extract, and ¼ teaspoon kosher salt together in a large bowl. Whisk in ⅓ cup all-purpose flour until smooth and free of clumps.
Whisk 4 large egg whites in a medium bowl until foamy. Add the remaining 1 tablespoon granulated sugar and whisk until soft peaks form, 2 ½ to 5 minutes. (Alternatively, mix with an electric hand mixer on medium speed.)
Transfer half of the egg white mixture into the egg yolk mixture and gently whisk together until just combined. Add the remaining egg white mixture and gently whisk until just combined; the mixture will be smooth and fluffy. Transfer to the baking pan, or evenly divide the batter between the ramekins (about ⅔ cup each; the ramekins will be full).
Pull the oven rack out about halfway and place the baking pan on it. Carefully and slowly pour enough of the hot water into the pan (do not let any water splash into the ramekins) so that it comes about halfway up the sides of the ramekins. Make sure the towel is completely soaked. Slowly push the oven rack back in.
For the cake: Bake until the cake is firm to the touch in the center and a tester inserted into the middle (inserting about halfway down) comes out clean, 35 to 40 minutes.For ramekins: Bake until the sides of each cake begin to slightly pull away from the ramekins, 32 to 34 minutes.
Remove the baking pan from the oven. Using tongs or oven mitts, carefully transfer the pudding cake(s) to a wire rack. Let cool for at least 15 minutes before serving. Serve dusted with powdered sugar and freshly grated lemon zest or topped with whipped cream if desired.
Tips
Make sure your oven is preheated. It will set your cake up for success by allowing it to rise to its fullest potential.
Line the bottom of your dish with a kitchen towel. It prevents your cake from moving around and helps the batter stay level to ensure even baking.
