I'm looking to try either the WASC or extender recipes because my two attempts at slab cakes have failed. I'm looking to do a 11x15x2 in pan cake.

my question is do I make the box mix as directed and THEN add the other ing. From the extender recipes???

also would 2 mixes + 2 extenders be enough for my cake?

thanks!

https://www.craftybaking.com/learn/baked-goods/wedding-cakes/charts/baking-time-and-batter-amounts-three-inch-deep-pans  16 cups of batter needed. You probably have enough with the 2 boxes of dry cake mix recipe. Which recipe are you using?

NO, the instructions on the box are NOT used. Just the ingredients in the recipe. The cake mix is used dry. Also, recipes call for 18.5 ox dry mixes that are now smaller. This may be making a difference as well. I use a kitchen scale. Sifting the dry ingredients helps too.

you need approx one fifth of another box of cake mix to get the 18.5 oz amount for each mix you use  so you could measure it out that way too