In a small saucepan, heat lemon juice and butter until butter is melted.
In a small bowl whisk together the egg yolks, sugar, and cornstarch until well combined and pale yellow.
Add a small amount of the hot lemon mixture to the egg mixture, whisking constantly.
Add the egg mixture to the remaining lemon mixture in the saucepan and whisk to combine. Cook over medium heat until thickened, stirring constantly, about 3 minutes.
Pour the lemon curd in a small bowl and let cool for 5 minutes.
Whisk in the powdered sugar and heavy cream until smooth. Add more heavy cream if the glaze is too thick.
Dip the tops of the donuts in the glaze.