In a sauce pan over low-medium heat, bring the milk to boiling.
While you wait for the milk to come to a boil, in a glass bowl, whisk together egg yolks, corn starch, sugar, vanilla, and lemon zest.
Once the milk is boiling, pour the milk into the bowl, whisking while you pour to create a smooth texture.
Next, using a hand strainer, pour the custard mixture through the strainer back into the sauce pan to remove any lumps. Heat over medium-low, whisking continuously, until the mixture thickens up well. Add butter and whisk until the butter is melted.
Remove the custard from heat, pour into a glass bowl and cover with plastic wrap. Place the plastic wrap directly on the top layer of custard to prevent it from hardening while it cools. Allow to cool before assembling the pastries.
Assemble the Pastries
Once the custard cools, line a sheet pan with parchment paper and preheat the oven to 400°F. In a small bowl whisk an egg and set aside to wash the pastry with later.
Using a cutting board or a clean counter, cut the puff pastry sheet into 12 even sized rectangular pieces. Then slice a rectangle inside of the puff pastry rectangles. Poke the center rectangle with a fork. (See above photos for example)
Spoon 2 Tbsp of custard in the middle rectangle and then brush the edges with the egg wash. Place raspberries top down on the custard. Bake pastries for 12-13 minutes or until the crust is golden brown.
Remove from the oven and allow pastries to cool. Sprinkle with confectioners sugar and serve.
Additional: You can also brush the tarts with raspberry jam for an extra shine. Warm 1-2 Tbsp of jam with 2 tsp. of water in a mini bowl for 30 seconds. Use a pastry brush to apply onto the raspberries.
Make sure the puff pastry completely thaws before you begin making the tarts.
Go for fresh raspberries.
Use a preheated oven.
You can glaze the tops of the raspberries with raspberry jam for an extra shine.
Make sure to whisk the custard until smooth in texture.
Allow the custard to cool completely before assembling the pastries.
You can prepare the custard the night before and refrigerate it for faster preparing the next day. Just make sure you cover the top layer of custard with plastic wrap.