Preheat the oven to 190 degrees C and grease and line your pan.
Sift the flours, cocoa powder and bicarb together into a medium sized bowl and then add in your chopped chocolate, stir to combine and then leave to one side.
Place the butter and sugars into a large bowl or the bowl of your stand mixer and beat until light and fluffy, about 2 minutes.
Add in the egg, vanilla extract and vinegar.
Mix until well combined and smooth, about 1 minute on med-high speed. Add in the red food colouring, little by little, mixing between, until you're happy with the colour.
Add in the flour mix and fold in on a low speed until combined. The dough will be stiff.
Place the dough into the prepared pan and smooth out with a spatula or wooden spoon.
Place in the oven for 20-25 minutes (mine took 20 minutes exactly) until, slightly risen, the top looks slightly crisp and an inserted skewer comes out clean.
Leave in the pan to cool for at least 30 minutes before transferring to a wire rack to cool completely.
Once cool slice in to 9-12 bars depending on how big you want them.
Bars will keep in an airtight container at room temperature for 4-5 days.