Butter is 80% fat. So if substituting butter with a 100% fat (such as vegetable shortening like Cookeen or Trex, or lard) use 20% less than the butter quanitity in the recipe.
1 medium UK egg white : 30ml : 2tbsp
Use 1/8tsp per 2 egg whites added at the foamy stage to aid whipping.
To convert a plain cake to chocolate, take away 20% of the flour, and replace with coco powder.