Can I use the cake extender recipe on a cake made from scratch. I need to make a red velvet cake and I don't want to make two batches.
i think you probably have to just double the batch...
A cake recipe is a balanced formula.
(That's why Baking is a science, and Cooking is an art).
Adding additional ingredients can unbalance the formula resulting in a cake failure.
If you don't want to make a second batch of scratch cake perhaps you'd rather make a doctored box cake recipe.
The WASC cake recipe has many flavor variations:
Doctored RV cake recipes: