Place the sifted Plain/AP Flour, Hazelnut Meal, Icing Sugar and Cacao Powder in the bowl of your food processor. Pulse for a few seconds to mix.
180 gr (1 cup + 3 tablespoon) Plain / AP Flour, 125 gr (1 1/4 cup) Hazelnut Meal, 20 gr (2 tablespoons) Unsweetened Cacao Powder, 100 gr (1 cup) Icing Sugar
Add the very cold butter cut into small cubes and blend until the butter turns into small crumbs that are evenly distributed around the dry ingredients (See note 1).
180 gr (3/4 cup) Unsalted Butter
Add the Egg Yolk and blend until a dough comes together (see note 2). Add the very finely chopped chocolate and mix until just combined.
1 Egg Yolk, 50 gr (1.7 oz) Dark Cooking Chocolate
Transfer the cookie dough over a lightly floured surface and lightly knead it to bring it together into a flattened ball. Place the dough between two sheets of baking paper and roll it with a rolling pin to be about 6mm (1/4 inch) thick (see note 3).
Place the flat rolled dough in the fridge and let it chill for at least 1 hour (or up to 24 hours), or until the butter has hardened again.
Preheat your oven on 160'C/325'F and prepare a large baking tray with a baking mat or baking paper.
Take the rolled dough out of the fridge and remove the top sheet of baking paper. With a Cookie Cutter the shape of your choosing (I used a 5 - 6 cm / 2 inch fluted cookie cutter), cut out the dough to make the shortbread cookies.
Re-roll any leftover dough between the two sheets of baking paper, chill it again until hard then cut out more cookies for a second batch.
Place the cookies over the baking tray (see note 4) and bake for 12 minutes. Take out of the oven, leave to cool down for 5 minutes on the tray then transfer over a cooling rack until completely cool.
To decorate
Melt the chocolate (either in the microwave in 30 seconds increments or over a double boiler) then dip half of each cookie into the melted chocolate. Gently shake each cookie over the bowl then scrape its bottom over the edge of the bowl to remove any excess chocolate (see note 5).
150 gr (5.3 oz) Dark Cooking Chocolate
Place over a sheet of clean baking paper and optionally, directly top with your choice of decoration while the chocolate is still melted. Let them cool down and set completely.
The large the crumbs of butter, the flakier the cookies will be. I would recommend to go for smaller crumbs as larger pockets of butter will make the cookies rise in the oven, which we want to avoid.
It might take a little while but it will get there! Stop as soon as the dough comes together to avoid over-mixing it. To know it is properly combine, press some of the crumbs in your hands. If they stick together, it's ready.
You can roll it thicker but will need to bake the cookie for a bit longer. I don't recommend rolling them thiner than this or they might burn too quickly and/or get dry.
You want to be quick here so that the dough remains very cold. If the dough starts to warm up (you will then notice that the cookies are harder to move around as they get very soft), place it back in the fridge for a few minutes.
If the chocolate starts to set and get harder, you can re-heat it up slightly to melt it again. You want to make sure the chocolate is very fluid to get the best results.