Melt coconut oil in a pot over low heat.
Combine cornflakes, coconut and sugar in a bowl (optional: crush cornflakes a bit with your fingers). Stir in flour, baking powder and sifted cocoa (you don't want lumps in the cocoa). Add melted coconut oil, mix well.
Press into a greased (I brushed with a bit of the melted coconut oil) 8inch/20cm square tin. Turns out I've given all my small tins to my kids, so I used a spring-form cake tin, which turned out to be genius - made the finished slice easy to remove whole before icing and cutting.
Bake 180C/350F for 20-25 mins. Will still be spongy in the middle if you touch it, but just starting to look slightly crispy on the edges. Cool in tin. Once cool, ice with basic chocolate icing, and lightly sprinkle with extra dessicated coconut. Cut into slices or squares. Store in an airtight container.
Icing: Icing sugar, 1-2 TBSP cocoa, a few drops of vanilla essence, 1 tsp dairy free marg (eg Nuttelex), enough boiling water to mix (add water a VERY little at a time). Stand a bread and butter knife in a cup of boiling water to heat, then use it to spread the icing - dipping back into the water as needed.
This was an experiment that turned out well. I knew I'd got it right when my (fussy) youngest daughter stopped in while I was out and ate some. I asked her the next day what she thought of it? She said "Yum - just like you always used to make!" She was surprised when I told her it was GF/DF as the old version certainly wasn't! I found that the day I made it, it tasted a little different than I was used to, but from the next day on it tasted great! So maybe store for a day before eating (if you can wait that long!)