ADoes anyone have an updated extender recipe for the 15 oz cake mixes?

get you a scale or you could measure it too --buy two cake mix and use the 18.5 ounces (or whatever it was) add enough to make up the difference -- just seal up the extra and use it for the next time --

best baking to you

I've used this one before, and it works nicely.  (From Food.com)

Cake Mix Extender:

3/4 cup flour

1/4 cup sugar

1 teaspoon baking soda

1/2 cup water

1/4 cup oil

1 egg

Mix all together and add to your already mixed cake mix batter.  Bake as usual.

For chocolate cake, I add a heaping tablespoon of cocoa powder.  For lemon cake, I replace 1/4 cup of the water with fresh lemon juice.