Preheat the oven to 180ºC/160ºC fan, and line a 9x9" square tin with parchment paper.
Add the butter, light brown soft sugar and white granulated sugar to a bowl and beat together until creamy - I use my stand mixer with the beater attachment!
Add in the egg, vanilla extract and red food colouring and beat again.
Add in the plain flour, cocoa powder, bicarbonate of soda, and sea salt and beat until a cookie dough is formed!
Add in the white chocolate chips and beat until they are distributed well!
Press the cookie dough into the lined tin.
Bake the cookie bars in the oven for 18-22 minutes, or until the top of the dough looks dry.
If you can't access one of the sugars, use all of the other one - i.e. all granulated, or all light brown sugar. Using other sugars can change the texture.