After much searching & a little tweaking, I came up with this. I'm very happy, these are buttery, rich, chewy, and sweet as anything. My favorite things to add are dried blueberries & white chocolate chips, although anything goes.
Preheat oven to 350. Mix flour, baking powder, baking soda and salt. Blend well, set aside.
Melt butter, add brown sugar & mix well. Cool slightly. Add to beaten eggs and vanilla, blend well. Add flour mixture a little at a time, mixing well, then chips, berries or whatever you're adding. Spread in well greased 9x13 pan, bake for 35 min.