Stir milk, vanilla extract, and butter in a saucepan. Cook over medium heat. While the milk is heating up, separate egg yolks from egg whites. Mix the egg yolks with 1/2 cup of sugar and corn starch in a bowl.
Once the milk reaches 140°F and starts to bubble on the edges, pour the egg yolk mixture into the milk and stir immediately. Stir continuously as the custard thickens. After about 5 minutes, once the custard thickens, remove it from heat.
Cool the custard at room temperature, then in the refrigerator for an hour. Or, to accelerate, place the saucepan with custard in an ice bath for 15 minutes.
Whip the heavy cream with 2 Tbsp of sugar until firm peaks form.
Wash the berries and cut the strawberries into bite-sized pieces.
Assemble the parfait. Add some custard, some berries, some whipped cream, some more berries, and then repeat until you reach the top of the glass. Enjoy immediately or refrigerate until ready to serve.