Preheat a fan forced oven to 180C / 355F (200C / 390F for no fan). Line two flat baking trays with baking paper.

Lay your frozen butter puff pastry on top of your baking trays and allow to thaw slightly. Use a fork to prick holes in both sheets of pastry. Cover with another sheet of baking paper and cover with another flat baking tray.

If using a fan forced oven bake for 40 minutes, if using no fan, bake for 50 minutes or until golden and crispy. Check it’s doneness at the 30 minute mark. Once baked, take out of the oven and allow to cool completely at room temperature.

Gently place an 8-inch square baking tin on top of one of the baked sheets. Use a serrated knife to gently score and trim the pastry to fit the base of the baking tin. Do the same with the other pastry sheet and the top of the baking tray.

Add the sugar and corn starch into a large heat proof bowl and whisk to combine. Then add the egg yolks and whisk until smooth. Set aside.

Place the milk into a non-stick large pot and begin heating on low heat, stirring occasionally. As soon as you see bubbles forming around the sides, take it off the heat.

Add about ¼ cup of the hot milk to the egg mixture while whisking. Slowly heat the egg mixture with the hot milk so that the eggs don’t heat too quickly and curdle. Add a little more milk and whisk. Once about half the milk has been added, you can add all of the milk. Whisk until combined.

Add the mixture back into the pot and begin stirring on medium heat. The mixture will begin to thicken after about 3-4 minutes. Once it’s completely thickened, continue stirring until it begins to bubble and steam. When it bubbles, stir for 3 minutes, then take off the heat and add the vanilla bean paste. Stir until combined.

While the mixture is hot, add the cold butter in 4 batches, stirring each time.

Line the bottom and sides of the baking tray with baking paper or double layers of plastic wrap, leaving some overhang and place the smaller pastry sheet in the bottom. Run the custard through a sieve on top of the pastry to remove lumps. Use a spatula to smoothen out flat. Place the second pastry sheet on top and cover with the baking paper or plastic wrap. Place it in the fridge to chill for a minimum of 5 hours, or overnight for best results.

Gently lift the vanilla slice out of the baking tin using the overhang. Place on a chopping board. Use a sharp serrated knife to gently score the pastry by running it on top of the vanilla slice. Cut the rest of the slice in one motion all the way to the bottom. Finish with a light dusting of powdered sugar and serve.