Heat oven to 400degreesF. Spray bottom only of 9-inch square pan with cooking spray. In medium bowl, stir muffin mix, pumpkin, water, oil and eggs just until blended (batter may be lumpy). Spread in pan.
Bake 17 to 21 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
In small bowl, mix cream cheese, powdered sugar, milk and vanilla until smooth; drizzle over bars. Sprinkle with walnuts. For bars, cut into 4 rows by 4 rows.