As the pastry is cooking place the cream, milk, sugar, butter, vanilla, lemon verbena leaves (and stalks if you have them) and lemon zest into a saucepan over very low heat. The low heat allows time for the verbena to infuse through the liquid. Heat until the liquid is hot, but not boiling. Remove from heat and allow to sit for 10 minutes. Strain the liquid, discard the leaves and return the liquid to the saucepan.