Heat the milk to a boil in a heavy pan. Cover and remove from heat.
Beat the egg yolks and sugar together in a bowl until light and thick.
Slow pour the hot milk into the egg mixture, whisking constantly. Turn the mixture to the sauce pan and cook over low heat, stirring with a wooden spoon until it thickens slightly and coats the back of the spoon.
Do not allow it to come to a boil or it will curdle.
Add in lemon extract, zest, and juice.
Allow the mixture to cool to room temperature and then refrigerate it, loosely covered with plastic wrap until chilled. At least 3 hours.
Freeze in the mixture in your ice cream maker.
Once it starts to set and thicken, add in the lemonade concentrate and continue mixing until set up.