Turn the oven up to 400 F.
Place egg yolks in a small bowl.
Combine sugar and cornstarch in a medium saucepan.
Add water and 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, lavender, and lemon juice until butter is melted.
Pour mixture into cooled pie crust.