When baking soda is paired with an acidic ingredient such as brown sugar, cocoa ... stored unopened for up to three years, but once the box is open, that shelf-life begins to dwindle.

Cut a large square of parchment the size to place on top of a baking tray. Put the brown flour and porridge oats into a large bowl and stir them together. Sieve the white flour, bread soda and ...