In a medium-sized bowl, whisk together the egg yolks with ¼ cup of the milk, sugar, vanilla, and cornstarch until well combined. The mixture should be smooth and creamy with a slight thickening as the cornstarch begins to activate.
In a saucepan, heat the remaining milk over medium heat until it’s hot but not boiling. You want it steaming and just about to bubble. Be careful not to let it boil, as that could curdle the milk.
Remove the saucepan from heat, and gradually pour the egg mixture into the hot milk, whisking continuously. This step is crucial; it helps temper the eggs and prevents them from scrambling.
Return the saucepan to the stove over low heat. Stir continuously until the mixture thickens and resembles a custard. It should coat the back of a spoon and have a smooth consistency.
Once thickened, remove the saucepan from heat and allow the custard to cool at room temperature. Stir occasionally to prevent a skin from forming on top.
Preheat your oven to 375°F (190°C). Prepare a baking sheet by lining it with parchment paper for easy cleanup.
Roll out the puff pastry on a floured surface into a rectangle about 18 inches long and 6 inches wide. You want it to be thin but not so thin that it tears.
Using a sharp knife, gently mark a shallow cut about ½ inch from the edge of the puff pastry to form a rim. Be careful not to cut all the way through; this will help the edges rise during baking.
Spread half of the cooled custard evenly onto the prepared puff pastry, keeping it within the marked rim. This will prevent it from overflowing while baking.
Arrange the sliced strawberries nicely on top of the custard, making sure to cover it beautifully. Sprinkle with sugar to enhance the sweetness of the strawberries.
Bake in the preheated oven for about 20 to 25 minutes, or until the puff pastry is golden brown and the edges are crisp. Keep an eye on it to avoid burning!
Once done, remove the tart from the oven and let it cool slightly. Dust with powdered sugar before serving for an elegant touch.
