Pre-bake pie crust slightly. For a frozen crust, I defrosted according to package directions and baked about 12 minutes at 350 F.
Turn oven to 400 F.
Place egg yolks in a small bowl.
Combine sugar, cornstarch, and water in a medium saucepan.
Heat over medium heat stirring constantly until mixture boils. Mixture will start to look clear and glossy, and will feel a bit thicker.
Continue stirring while the mixture boils for one minute.
Pour approximately half the sugar/cornstarch mixture into the bowl with the egg yolks and stir till combined.
Pour the egg yolk mixture back into the saucepan with remaining sugar/cornstarch mixture, stirring constantly over medium heat for an additional two minutes. Mixture will still be boiling.
Remove from heat and mix in the butter, prosecco, Aperol, and sparkling water, stirring until butter is melted and liquids are mixed in.
When combining the hot mixture and the egg yolks, you want to be sure the eggs don't get too hot too fast or they’ll cook, and you don’t want scrambled eggs in your pie! Adding a little bit of the hot mixture to the eggs while stirring, before adding the eggs to the pot will heat them slowly and will prevent them from cooking.