2. While you wait for the chocolate to cool, position a shelf in the middle of your oven and turn the oven on to fan 160C/conventional 180C/gas 4 (most ovens take 10-15 minutes to heat up). Using a shallow 20cm square tin, cut out a square of non-stick baking parchment to line the base. Now tip85g plain flour and 40g cocoa powder into a sieve held over a medium bowl, and tap and shake the sieve so they run through together and you get rid of any lumps.