1.Preheat the oven to 350 F. Microwave 2 tablespoons butter in a large soufflé dish until melted, use a brush to grease the entire dish. Add 1/4 cup of sugar and roll the dish, making sure to cover all the interior surfaces. Set aside.
2.Bring 1 cup of milk to just steaming in a medium saucepan set over low-medium heat.
3.In a large bowl stir together 1/3 cup sugar, flour, lemon zest, 1/3 cup milk, and 4 egg yolks until it forms a smooth batter.
4.Slowly whisk half of the hot milk into the batter, making sure to combine the ingredients until they are completely smooth. Add the tempered batter back to the hot milk in the pan and bring the mixture to a simmer, stirring constantly. Cook the mixture until it has thickened, for about 1 minute.
5.Stir 1 1/2 tablespoons butter into the mixture and allow it to cool at room temperature for 10 minutes then stir in vanilla extract.
6.In the bowl of a stand mixer, beat the egg whites on medium-high speed until they become foamy, and then add 2 tablespoons sugar. Continue beating the egg whites on high speed until they hold stiff glossy peaks.
7.Gently stir 1/3 of the egg whites into the vanilla mixture, then carefully fold in the remaining whipped egg whites. The mixture should be evenly colored and light and bubbly, without egg white streaks or marbling.
8.Spoon the soufflé mixture into the prepared dish. Bake for 25 to 30 minutes until the soufflé has risen with a crusty exterior. Take out of the oven and dust with powdered sugar, serve immediately.