In a large bowl, cream the butter and brown sugar until light and fluffy.
Combine the flour, oats, baking powder, baking soda and salt; gradually add to creamed mixture (mixture will be crumbly).
Set aside 2 cups; pat remaining crumbs into a greased 13-in. x 9-in. baking pan.
Bake at 350° for 15 minutes. Cool.
Meanwhile, in a small saucepan, combine filling ingredients.
Bring to a boil; cook and stir for 5-8 minutes. Pour over crust; sprinkle with reserved crumbs.
Bake additional 15 minutes.
Source:
tasteofhome.com