Preheat oven to 190C/170C fan/gas 4. Grease two baking trays with butter.
Cream together the butter, sugar and vanilla extract until light and fluffy. Add the egg, red colouring and buttermilk and beat until just combined.
Sift the flour, cocoa and bicarbonate of soda into the bowl and add the vinegar. Beat until combined, then stir through the white chocolate chunks.
Place spoonfuls of cookie dough onto the baking trays, allowing room for the cookies to spread, four on each tray should work well. Flatten each cookie a little with the back of the spoon, and bake for approximately 13 minutes.
Leave the cookies to set on the tray for five minutes before placing on a wire rack to cool.