Preheat oven to 350º and line two baking sheets with parchment paper, and line a 9×13-inch baking dish with parchment paper, leaving a 2-inch overhang on both sides.
Unroll puff pastry sheets and place onto lined baking sheets, using a fork to gently prick all over the dough, without piercing the bottom.
Optional: top each sheet of puff pastry with parchment paper, then top with another baking sheet, acting as a weight, so puff pastry won’t inflate.
Place baking sheets in oven and bake for 20-22 minutes, or until golden brown. Remove from oven and let cool completely.
Optional: use a serrated knife to trim edges off puff pastry so that it fits into 9×13-inch baking dish. Place one sheet of puff pastry
In a medium saucepan, whisk together milk, cream, butter, vanilla and sugar over medium heat, and cook until little bubbles appear around the edge of the pan, before it comes to a boil.
Whisk together flour and water to create a slurry, then whisk into hot milk mixture and remove pan from heat.
Whisk a little of hot milk mixture into beaten egg yolks to temper the yolks, then, while saucepan is still off the heat, whisk yolks into mixture until combined.
Return saucepan to the heat and bring to a boil. While whisking continuously, cook for 30-60 seconds, or until custard thickens.
Pour thickened custard over puff pastry in 9×13-inch baking dish, then top with second sheet of puff pastry. (Trim edges to fit dish.) Gently press down so pastry adheres.
Refrigerate baking dish until custard is set. 3-4 hours.
When ready to serve, dust with powdered sugar, slice and serve.
