Heat oven to 350°F. Grease 13x9-inch baking pan.
In a large mixing bow, combine all oatmeal layer ingredients except the butter. Cut in 2/3 cup butter with pastry blender or fork until sandy. Reserve 1 1/4 cups crumb mixture for topping. Press remaining crumbs into prepared pan and bake 10 minutes.
Combine 1/3 cup butter and chocolate in bowl. Microwave until chocolate is melted. Allow to cool slightly.
In a the bowl of a mixer, combine granulated sugar, eggs, water and vanilla and beat at medium speed until well mixed. Add melted chocolate mixture; continue beating until well mixed. Add 1 cup flour and baking powder; beat at low speed until well mixed.
Carefully spread brownie layer evenly over warm pre-baked crust; top with reserved crumb mixture.
Bake 18-22 minutes or until edges are set. (Do not overbake.) Cool completely before cutting into bars.
Recipe adapted from Land O Lakes