Preheat the oven to 350 degrees F (175 degrees C).
In a saucepan, combine white sugar, brown sugar, flour and salt. Pour in milk and add margarine. Heat, stirring constantly, until sugar dissolves and margarine is melted. Remove from heat and set aside to cool.
In a large bowl, whisk egg yolk, egg and vanilla until smooth. Whisk in cooled sugar mixture. Pour into pie crust.
Bake for 25 minutes in the preheated oven, then reduce the temperature to 250 degrees F (120 degrees C) and bake for an additional 25 minutes, or until top is puffed and edges are golden brown. Allow to cool before slicing.