In a saucepan, bring the water, lemon juice, salt, 1 cup sugar and butter to a boil over medium-high heat.

In a small bowl, whisk together the cornstarch and 1/2 cup cold water until dissolved. Pour into the boiling mixture and reduce heat to low. Cook for 2-3 minutes until thickened, stirring constantly.

In a separate bowl, lightly beat the egg yolks. Temper the yolks by whisking in a few spoonfuls of the hot lemon mixture. Then pour the tempered yolks back into the saucepan and cook for 1 minute more. Remove from heat and stir in the lemon zest. Let cool for 15 minutes.

Using a hand mixer or stand mixer fitted with the whisk attachment, beat the egg whites on high speed until foamy. Gradually add in the remaining 1/2 cup sugar and beat until stiff, glossy peaks form, about 3-4 minutes.

Gently fold the whipped egg whites into the lemon custard base in two additions until fully incorporated.

Pour the chiffon filling into the pre-baked pie crust and smooth the top. Gently cover with plastic wrap or wax paper and refrigerate for at least 2 hours until completely set.

When ready to serve, top with sweetened whipped cream and garnish as desired with berries or lemon slices.