1Heat oven to 350°F. Spray 13x9-inch pan with cooking spray. In medium bowl, mix flour and brown sugar. Using pastry blender, cut in butter until mixture looks like coarse crumbs. Stir in oats and walnuts. Press in bottom of pan; bake 15 minutes.
2In large bowl, beat filling ingredients with electric mixer on medium speed until well blended. Pour over hot base. Bake 20 to 25 minutes or until set and dry in center.
3Meanwhile, in small bowl, mix topping ingredients. Drop mixture by spoonfuls over pumpkin layer; spread evenly over hot filling. Bake about 5 minutes or until topping is set. Cool completely, about 2 hours. Cut into 4 rows by 4 rows. Store covered in refrigerator.