Preheat the oven 200˚C. Line a baking tray with a sheet of baking parchment and set aside. Line the base and sides of a 20cm square high sided baking tin with parchment paper or cling film and set aside.
Prepare the pastry layers by splitting the block of puff pastry in two and placing one half in the fridge. Roll out the remaining pastry to roughly 25cm square.
Transfer the pastry to the prepared baking sheet and place a second baking tray on top- this will allow the pastry to rise slightly and bake evenly. Place in the oven to bake for 25 minutes until the pastry is golden brown. Transfer the cooked pastry to a wire rack to cool completely. Repeat the process with the remaining pastry.
For the custard filling, place the gelatine sheet in a small bowl of cold water and allow to soften while you prepare the custard. In a saucepan, whisk together the egg yolks, caster sugar, lemon zest, vanilla bean paste & cornflour.
Slowly whisk in the milk and double cream until smooth and then place over a medium high heat and bring to a steady simmer. Whisk continuously until the custard thickens. It should be quite thick and spreadable. Remove from the heat and squeeze any excess water from the gelatine sheet before mixing vigorously through the custard until it’s well combined.
Using a big, sharp knife and a ruler – or the base of your tin if it is removable – trim one pastry sheet to a shade under a 20cm square, so it will sit inside the tin.
Spread over the custard as evenly as you can. Trim the second pastry sheet in the same way, but then divide the square into eight 10 x 5cm rectangles (so it will be easier when you cut the slices later). Sit the pieces of puff pastry on top of the custard, putting them back together a bit like a jigsaw puzzle. Gently press down on the pastry so it doesn’t break but sticks to the custard, then cover and chill for 4 hrs or so, but better still overnight.
Meanwhile, make the icing. In a small bowl, mix the icing sugar with the lemon zest and a squeeze of juice, until smooth and just spreadable (not too runny). Remove a spoonful of the mixture to another small bowl and stir in a little yellow food colouring.
Spread the white icing over the pastry. Using a spoon, drizzle lines of yellow icing over the white icing at even intervals. Drag a toothpick through the yellow lines to create a feathered effect.
Chill for 1 hour before cutting into slices to serve.
I have always had a fondness for a custard slice, wobbly vanilla studded custard sandwiched between flaky golden puff pastry and sweet icing to finish - what’s not to love.