In a small saucepan over medium heat, slowly heat the milk and lemon verbena leaves until steaming but not boiling. Turn off the heat and cover to steep for 1 hour.
Strain out the leaves and set the milk aside.
In a separate bowl, whisk together cornstarch and sugar. Add eggs and whisk until smooth. Set aside.
In a saucepan, heat the milk and vanilla bean paste over medium-high heat until just below boiling. Stir occasionally.
Slowly add 1/3 of the hot milk to the egg mixture, whisking constantly to avoid cooking the eggs. Pour the egg-milk mixture back into the remaining hot milk. Whisk over medium heat until thickened, just below boiling. Remove from heat and strain through a sieve into a bowl.
Cool for 5-10 minutes, stirring occasionally, then mix in butter one piece at a time until smooth.
Cover the pastry cream with plastic wrap pressed directly onto the surface and chill in the refrigerator.