Preheat oven to 350 degrees. Line 9x9 with foil & spray with non-stick spray. Mix dry ingredients & set aside. With mixture with paddle attachment, mix butter & sugars. Add vanilla $ egg. Mix well. Add dry ingredients & mix well.
Press 1/2 of the batter into pan. Sprinkle with chips & Oreos. Drizzle with condensed milk. Drop remaining dough by spoonfuls over top & spread with fingers. Some of the filling will peak through. Try & seal edges as much as possible.
Bake for 27-30 min. or till top loses its glossy sheen & looks dry on top. Center may jiggle a little.