In a medium saucepan, whisk together the sugar, flour, cornstarch, and salt.
Add Liquids:
Gradually stir in the water, lemon juice, and lemon zest until the mixture is smooth.
Cook:
Cook the mixture over medium-high heat, stirring constantly, until it thickens and comes to a boil. Continue to cook and stir for 2 minutes more, then remove from heat.
Temper the Egg Yolks:
In a small bowl, gradually whisk about 1/2 cup of the hot lemon mixture into the beaten egg yolks, then immediately whisk the egg yolk mixture back into the remaining lemon mixture in the saucepan.
Finish Cooking:
Return the saucepan to the heat and bring to a gentle boil, cooking for an additional 2 minutes while stirring constantly.
Add Butter:
Remove from heat and stir in the butter until melted and well combined.
Cool and Serve:
Pour the custard into a bowl, cover with plastic wrap directly on the surface to prevent a skin from forming, and let it cool to room temperature. Chill in the refrigerator for at least 2 hours before serving.
Serving Suggestions:
Serve chilled in individual cups or bowls.
Garnish with a dollop of whipped cream and a lemon slice or zest.