Double dark chocolate shortbread cookies

Today I made two batches of shortbread cookies - the first were peanut butter milk choc chip ones, and these are double dark chocolate - loaded with cocoa powder and chunks of dark chocolate. I used Cadbury's Bournville, which is my favourite dark chocolate - so dark and bitter it makes quite a grown-up, truffly cookie. These are absolutely amazing, I promise!

Recipe adapted from The View from Great Island

Ingredients
4oz butter, softened
2 tsp vanilla extract
5oz plain flour
2oz icing sugar
1oz cocoa powder
3oz dark chocolate, chopped

Method

1. Cream the butter and vanilla.
2. Sift in the flour, icing sugar and cocoa and beat until a smooth dough is formed.
3. Stir in the chopped chocolate.
4. Shape the dough into a log, wrap in baking paper and twist the ends. Refrigerate for an hour, or overnight. 
5. Pre-heat the oven to 170°C and line two baking sheets with baking paper.
6. Remove the dough log from the fridge and use a sharp serrated knife to cut into ¼ inch slices. (If they crumble at all when slicing, don't worry - just squidge them back together.)
7. Place the cookie slices on the baking sheets, leaving space between them to spread (probably about 6 per sheet).
8. Bake for 12 mins approximately.
9. Leave to cool on the baking sheets from 5 – 10 minutes, then transfer to a wire rack to cool completely.