Preheat the oven to 350 degrees and line a baking sheet with parchment paper.

Place one sheet of the puff pastry onto the prepared baking sheet. Prick it all over with the tines of a fork.

Add a layer of parchment paper to the top of the laid-out puff pastry, and place the second sheet of puff pastry on top of the piece of parchment paper. Prick this second layer with a fork as well.

Lay another sheet of parchment paper on top of the second puff pastry and place a second baking sheet on top of everything to weigh down the puff pastry and keep it from rising too much when baked.

Bake for 25-30 minutes or until the puff pastry is cooked through. Remove the sheets from the oven and allow them to cool.

Once cool, cut each of the sheets of puff pastry to fit into a 9x9 baking dish.

In a large pot over medium heat, add the butter, milk, heavy whipping cream, and sugar.

In a medium bowl, whisk together the flour and water, and then pour the mixture into the pot.

Stir the custard mixture until the butter is melted and the sugar has dissolved. At that point, remove your pot from the heat. Add the vanilla and combine it well.

Whisk together the egg yolks in a bowl, and pour in a little of the mixture from the pot, whisking continuously as you do. This will temper the eggs to give your custard a thick and silky texture.

Once the egg yolks have been tempered, whisk them into the pot and return the pot to the heat.

Bring the entire mixture to a boil for 1 minute, remove it from the heat, and allow the custard to cool and thicken.

Lay a layer of cooked puff pastry in the bottom of a well-greased 9x9 baking dish and top it with the custard.

Add another layer of puff pastry on top.

Chill the baking pan in the refrigerator until the custard has set. Once ready, dust the top of the bars with some powdered sugar if desired, cut, and enjoy!