Weight or measure out all the dry ingredients first - the flour, poppy seeds, icing sugar and large pinch of salt. Place in a large bowl and mix all together.
Make sure your butter is softened before adding in to the flour mixture.
Add the butter, egg yolk and vanilla extract to the flour mixture and mix all together. I start with a wooden mixing spoon, but then I also use my hands to get everythig mixed in perfectly. You should end up with a soft, but firm (ish) dough.
Roll out the dough between two sheets of baking parchment to about 8 mm thickness.
Place on the baking tray with the baking parchments and leave in the fridge for about 20-30 minutes to firm up.
Preheat your oven to 160C or 180 Fan, 350 F or Gas mark 3.
Cut out your cookies shapes. I've used 5 cm cutter.
Use up all the dough and when you are re-rolling your dough try to handle it as little as you possibly can.
Place your cookie shapes on a baking tray lined with baking parchment, making sure you leave a gap between them. They are not going to rise or expand very much, but you never know!
Bake for 10 min or until they are slighly golden. They shouldn't change their colour too much - if they do, you baked them a little too long.
Leave to cool down on a cooling rack.
To make the lemon icing
Mix together the icing sugar, lemon juice and egg white to make a runny icing. You can use spoon to spread the icing on the cookies or use a piping bag to make a drizzle effect.
Sprinkle extra poppy seeds on the top or lemon rind or similar decorations.
Leave to set completely (this could take about 24 hrs, if you are thinking of packing your biscuits away, but you can eat them straightaway).