Proportions for prunes as replacement for butter
I've had very good success from Sandra Woodruff's Secrets of Fat Free Baking (no idea if it's still in print?)
As per her instructions:
"The fat in baked goods can be replaced by half as much prune puree. When using Prune Butter, replace with an equal amount..
To give your cookies the best texture, replace one-half to 3/4 the fat with Prune puree. Before baking, flatten the cookies slightly to aid spreading.Cook at the same temp. and time as recipe calls for."
Cake, Muffin and Quick Bread recipes have different instructions/proportions.
3 oz pitted prunes (1/2 c)
1 c water
2 t lecithin (helps improve texture)
8 oz pitted prunes (1 1/3 C)
6 t water
She's got an Oatmeal cookie recipe in here if you wish
For cookies, I think it works best to replace no more than half the butter with fruit substitutes, so that's what I'd suggest. Two things to bear in mind: First, don't use prune puree in your oatmeal cookies unless you want them to taste strongly of prunes; use unsweetened applesauce, instead (prunes work better with strong flavors like chocolate). Second, using these substitutions generally results in a caky texture.