Equal parts sweet, tart, and herbaceous, we love this pastry cream as a filling between delicate layers of our Lemon-Thyme Tower Cake.
In a medium saucepan, combine milk, thyme, lemon zest, and 1/2 cup sugar. Bring to a boil over medium-high heat. Remove from heat, cover, and let stand 15 minutes.
Prepare an ice bath. In a medium bowl, combine cornstarch, flour, remaining 1/2 cup sugar, and salt. In another bowl, whisk eggs until smooth. Add cornstarch mixture to egg mixture; whisk until well combined.
Strain the warm milk mixture; discard solids. Return milk to saucepan; bring to a boil. Slowly pour into egg-cornstarch mixture, whisking constantly.
Transfer new mixture to saucepan set over medium-high heat. Bring to a boil, whisking constantly, until thickened, about 2 minutes. Remove pan from heat, and transfer mixture to a large heat-proof bowl.
Whisk in butter; set bowl in ice bath, stirring occasionally, until chilled. Lay plastic directly on surface of pastry cream to prevent a skin from forming; chill until ready to use.