with a little bit of the milk. add in sugar and corn flour, mix well.
in a saucepan, bring the remaining milk to a simmer. remove from heat and add the hot milk gradually to the yolk mixture, whisk to combine.
pour the mixture over a sieve and return it to the saucepan.
bring it to boil on low heat, stirring constantly with a wooden spoon to prevent the mixture from turning lumpy or burnt at the bottom of the pan. when it boils, continue to stir constantly for another 1 minute.
remove mixture from heat and whisk in the butter, lemon zest and lemon juice.
pour the filling into the "cheater" pastry crust and 