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

To prepare the pastry - 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.

To bake the pastry - If using a fan forced oven bake for 40 minutes, if using no fan, bake for 50 minutes or until golden and crispy. Once baked, take out of the oven and allow to cool completely at room temperature.

Cutting the pastry to size - Gently place a 8-inch baking tin on top of one of the baked sheets. Use a serrated knife to gently score and cut down the pastry to the size of the bottom of the baking tray. Do the same with the other pastry sheet and the top of the baking tray. Sometimes baking trays can be a different size on the bottom and top. If you want to be extra, you can use a ruler to measure out and cut to size that way.

Combine the dry ingredients - 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.

Heat the milk - 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.

Combine hot milk and egg mixture - Add about ¼ cup of the hot milk to the egg mixture while whisking. You want to 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.

Cook the custard - 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 2 minutes, then take off the heat and add the vanilla bean paste. Stir until combined.

Adding the butter - While the mixture is hot, add the cold butter in 4 batches, stirring each time. You want to add the butter slowly so that it has time to mix and melt into the custard without splitting it. This is also why we use cold butter. It melts slower.

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. Use a spatula to smoothen out flat. Place the second pastry sheet on top and cover with the baking paper or plastic wrap. Place in the fridge to chill for 4 hours or overnight.

Gently lift the vanilla slice out of the baking tin using the overhang. Place on a chopping board.

You want to use a sharp serrated knife. That’s important. Gently score the pastry by running it on top of the vanilla slice. You can use a metal ruler to help guide where to cut. Once you score through the top pastry layer cut the rest of the slice in one motion all the way to the bottom. Carefully wipe the knife clean before making the next cut. Finish with a light dusting of powdered sugar and serve.