HEAT oven to 350 degrees F. Spray bottom only of 13 x 9-inch pan with no-stick cooking spray. Melt butter in medium saucepan; stir in flour until smooth. Add brown sugar and 2 eggs; mix well.
COOK over medium-low heat 5 minutes, stirring constantly. Remove from heat; stir in vanilla and pecans. Set aside.
COMBINE all brownie ingredients in large bowl; beat 50 strokes with spoon. Spread in prepared pan. Spoon topping evenly over top.
BAKE 30 to 35 minutes or until set. Cool completely. Cut into bars.
High Altitude (above 3500 ft.):
ADD 1/4 cup flour to dry mix. Bake as directed above.