Sift the dry ingredients (flour, sugar, baking powder) into a mixing bowl.
Add the butter (grated or cut into small pieces) and rub it into the flour with your hands like when making crumble.
Add the yolk and lemon zest (optional) to the crumbly mix and bring all together, forming a dough. Don't knead the dough too long.
Wrap the sticky dough in cling film and let it cool in the fridge for at least 1 - 2 hours.
Line the baking tray with baking paper. Preheat the oven to 180 C/ fan 160 C.
Divide the dough in half and leave the other half in the fridge.
Generously dust the kitchen worktop with plain flour. Take the first half of the dough, let it soften for 5 minutes at room temperature, and roll it into a 3 mm thick sheet. Tip: If the dough is still too hard to roll, make it more playable by squashing it in your hands for a short while (like play dough) - to remove the stiffness. Cut out the first batch of biscuits. Start with the one with a little cut-out window. Those will form the top of the sandwich biscuit.
With a cookie cutter, cut the shapes from the dough and transfer them onto the tray. Once they are all on the tray, cut the little openings from the middle of each biscuit. Tip: Remove the small piece of dough using a toothpick or the tip of the knife. Bake biscuits for around 9 - 10 minutes or until the edges start to brown.
Remove cooked biscuits from the tray and let it cool before baking the second batch (or use another tray).
Roll the other half of the dough into a 3 mm thick sheet. Cut out the shapes (these are for the bottom of the biscuit and don't have a small cut-out) and transfer them onto the sheet.
Bake the second batch of biscuits for 9 - 10 minutes.
Note: Don't waste the leftovers. Pat the off-cuts together and re-roll the dough once again, repeating the rolling, cutting, and baking process.
Wait for all the baked cookies to cool down and sandwich two pieces together with a small amount of jam (don't forget, one of them has a little cut-out).
Tip: Sandwich together the sides that were touching the tray (they are usually flat
Sprinkle sandwiched biscuits with icing sugar.