Mix the butter, sugar and vanilla essence using a whisk until creamy.

Add the egg and whisk again.

Change to a spatula and mix in the flour and salt.

Divide the dough in half and add the cocoa powder to one of them.

Cut 4 pieces of baking paper and fold them to make a rectangle. Put one of the doughs in between two of them and fold in the sides again to form the rectangle. Roll out using a rolling pin until nice and even and repeat with the other flavour.

Let them rest for 1 hour in the fridge. If it's there longer than that and it goes too hard then leave it outside for a while before going to the next step.

Remove one of the papers from both flavours and put one on top of the other. Use a rolling pin to roll them out a bit and stick them together.

Roll the dough using your hands and the paper.

Make a double layer of cling film (this prevents it from sticking to itself) and use it to wrap the dough roll we made. Make sure it's nice and tight.

Take to the freezer for at least 4 hours.

Cut 3mm slices with a sharp knife.

Take them to a preheated oven at 180C for 10-12min or until they start to go golden.