Prepare the Puff Pastry: (See Notes to Bake Ahead)

Cup of Yum
1. Preheat your oven to 400°F (205°C) and grease an 8x8-inch baking dish. Set aside.
2. Roll out one sheet of thawed puff pastry on parchment paper until it’s 2 inches larger than the baking dish. Place it on a baking sheet and prick the surface all over with a fork.
3. Bake for 10 minutes. When it starts puffing up, remove it from the oven and place a second baking sheet directly on top to flatten it. Bake for another 10 minutes or until golden brown.
4. Repeat the process with the second sheet of puff pastry. Let both sheets cool completely, then trim them to fit snugly into the baking dish.
5. Place one puff pastry sheet into the base of the prepared dish, and set the other aside.

Make the Custard Filling: (Make & refrigerate up to 2 days ahead)

1. Whisk together the egg yolks, sugar, cornstarch, and salt in a heatproof bowl. Set aside. 
2. In a medium saucepan, whisk together the milk, cream, and vanilla over low heat. Heat until it just starts to simmer.
3. Slowly whisk ¼ cup of the hot milk mixture into the egg yolk mixture to temper the eggs. Repeat this step with ¾ cup more milk, adding it in three ¼-cup intervals and whisking well each time.
4. Place a fine-mesh sieve over the saucepan with the remaining hot milk. Pour the tempered egg mixture through the sieve to catch any cooked bits.
5. Return the saucepan to low heat, stirring constantly for 8–10 minutes, until the mixture thickens to a pudding-like consistency and coats the back of a spoon.
6. Remove from heat and stir in the butter until melted.
7. Transfer the custard to a heatproof bowl and let it cool for 5 minutes. Cover the surface with plastic wrap to prevent a skin from forming, then refrigerate until completely chilled, about 3–4 hours. Tip: You can make the custard up to 2 days ahead of time and store it in the fridge until ready to use.

Make the Chocolate Ganache:

1. In a microwave-safe bowl, combine the chocolate chips with the whipping cream. Microwave in 20–30 second intervals, stirring well after each heating, until the chocolate is melted and the mixture is smooth. This should take about 60–90 seconds total. Alternatively, stirring constantly, you can melt the mixture over low heat on the stovetop.
2. Once melted and smooth, add the softened butter and stir until fully incorporated. Then mix in the vanilla extract and a pinch of salt. Allow the ganache to cool slightly before using. 

Assemble the Cake:

1. Spread the chilled custard over the puff pastry base using a silicone or offset spatula.
2. Place the second sheet of puff pastry on top of the custard layer.
3. Pour and spread the ganache evenly over the top pastry layer, making sure to coat all the corners. 
4. Chilling it overnight (8-12 hours) is even better for optimal results, as it enhances the flavor and makes slicing cleaner and easier. But if you don’t have that much time, chill for at least 4 hours before serving.