In a small saucepan, bring the milk and lemon zest to a boil over medium heat. Remove the pan from the heat and allow the mixture to cool for 5 minutes.
In a medium bowl, mix together the egg yolks, sugar and flour (mixture will be thick). Pour the milk mixture into the bowl and whisk until smooth. Return the mixture to the saucepan and whisk constantly over medium-low heat until smooth and thick, about 6 to 7 minutes. Remove the pan from the heat and stir in the butter. Whisk in the vanilla and lemon juice until smooth. Transfer the pastry cream to a clean bowl and lay a piece of plastic wrap directly onto the surface. Refrigerate for 2 hours.
Using a paring knife, cut the tops off of the profiterole shells and reserve. Using a small dessert spoon or a pastry bag fitted with a plain tip, fill the shells with the pastry cream. Cover with the profiterole tops and arrange them on a serving platter.
Recipe courtesy of Giada De Laurentiis