Make the Dough: In a large bowl, combine the 250g plain flour, 75g sifted icing sugar, and a pinch of salt. Add the 125g of cold, cubed butter. Using your fingertips, rub the butter into the flour mixture until it resembles fine breadcrumbs.

Bring it Together: In a small bowl, lightly whisk the egg yolk and 1 tsp of vanilla extract. Pour this into the flour mixture and use a knife or spatula to stir until it starts to clump together. Gently bring the dough together with your hands, forming a smooth disc. Avoid overworking it.

Chill the Dough: Wrap the dough disc tightly in cling film and place it in the refrigerator to chill for at least 1 hour, or up to 2 days. This step is essential for preventing the cookies from spreading.

Prepare the Filling: While the dough is chilling, make the filling. In a small bowl, mix together the 70g dark brown sugar, 1 tbsp plain flour, and 1 tsp cinnamon. Pour in the 2 tbsp of melted butter and stir until you have a thick, damp paste.

Roll and Cut: Once chilled, unwrap the dough and place it on a lightly floured surface. Roll it out to approximately 3mm thick. Using a sharp knife or a rectangular cookie cutter, cut out 24 rectangles, each about 5cm x 8cm. I find that using a ruler helps keep things neat.

Assemble the Cookies: Arrange 12 of the rectangles on a baking tray lined with baking parchment. Spoon about a teaspoon of the brown sugar filling into the centre of each, leaving a 1cm border around the edges. Lightly brush the border with a little water.

Seal and Crimp: Carefully place the remaining 12 rectangles on top. Gently press down around the filling to push out any air, then use the tines of a fork to crimp and seal the edges all the way around. Prick the top of each cookie a couple of times with the fork.

Final Chill & Bake: Place the entire baking tray in the freezer for 15 minutes. This helps the cookies hold their shape perfectly. Meanwhile, preheat your oven to 180°C (160°C fan). Bake the chilled cookies for 12-14 minutes, or until the edges are a light golden brown.

Cool Completely: Let the cookies cool on the baking tray for 5 minutes before carefully transferring them to a wire rack to cool completely. They must be fully cool before you add the icing.

Decorate: To make the icing, mix the 100g sifted icing sugar with 2 tsp of milk and the ½ tsp vanilla extract until you have a smooth, spreadable consistency. Add a little more milk if it's too thick. Spread a thin layer of icing over each cookie and immediately top with rainbow sprinkles before it sets.