In a food processor, add the cubed cold butter and flour and sugar. Blitz a few times to start the mixing.
Add the egg, vanilla and salt and blitz again until the dough starts forming. It’s done as soon as it is smooth and holds together.
Pop the dough in a plastic bag in the fridge for 30 minutes to rest before rolling out.
On a floured surface, cut the dough in half and roll out each piece on a sheet of baking paper to approx. 30 x 30cm. Transfer the pastry and baking sheet to a baking tray.
Pop both trays in the fridge again for 10-15 minutes.
Turn the oven to 180C/350F/Gas 4
Prick holes in the dough using a fork to prevent the dough from rising or misshaping during baking. Bake until golden (10-12 minutes, depending in your oven), then remove from the oven and leave to cool for just a few minutes.
Meanwhile, prepare your icing: Add the icing sugar to a bowl and add 2-4 tablespoons of hot water – you may need more water than this, but start with two. Stir, adding more water if needed, until you have a thick icing with the texture of syrup (i.e. not too runny).
On the still slightly warm pastry, add the jam and spread carefully and evenly all over one fo the pieces. Add the second pastry on top so it lines up (you may need to use the baking tray to guide it on so it does not break – this bit is tricky).
Carefully, using a spatula, smear the icing across the top. If your icing is too thick, it won’t work – and too runny, it will spill everywhere, so test a little corner first and adjust accordingly.
As soon as you have spread your icing, add your toppings.
Using a very sharp knife, cut into 12-16 pieces. You may find it easier to cut it once it has all cooled down and the icing has set. Although some swear by cutting when hot, we do find it easier to do when cold, using a good knife.Carefully, using a spatula, smear the icing across the large cake. If your icing is too thick, it wont work - and too runny, it will spill everywhere, so test a little corner first and adjust accordingly.