Sift together the flour and cacao/cocoa powder
In the bowl of a stand mixer fitted with paddle attachment, beat the butter on medium-low until smooth (or mix by hand with a wooden spoon if you have the time and energy). Add the sugar and continue to beat until butter is smooth. Add the egg yolk, salt and pepperberry mix and vanilla and mix until smooth
Add the dry ingredients 1/3 at a time, beating on low speed only until each lot is fully incorporated
Turn out the dough on a floured work surface. Divide the dough in half and roll into two 2.5-3 cm thick logs (about 15 cm long). Roll the logs back and forth under your palms to get a good shape. Once formed, wrap the logs in plastic wrap and refrigerate for at least four hours. TIP: Placing your plastic wrapped shortbread dough rolls inside a paper towel roll (or similar) will help them to keep shape and not flatten on one side while they rest in the fridge
To bake, preheat your oven to 175C (350F)
Line two baking sheets with parchment/baking paper
Remove the dough rolls from the fridge, brush each roll with egg wash and sprinkle with the pepperberry salt blend (or salt and black pepper) and sugar mixture
Use a sharp knife to cut logs into rounds and place them on the baking sheet, leaving approx 3 cm between them - they will spread a little
Bake for 14-16 minutes, rotating the sheets from top to bottom and front to back at the 7-minute mark
Remove from oven and let sit on the tray for a few minutes – they will firm up enough to lift without breaking in this time. Transfer to wire cooling rack to cool to room temperature