Heat oven to 350°F. Spray 9-inch square pan with nonstick cooking spray. In medium bowl, combine brown sugar and egg product; beat until light. Slowly add oil, beating until well blended. Stir in 2/3 cup milk.
Add whole wheat flour, all-purpose flour, baking powder, baking soda and salt; stir just until dry ingredients are moistened. Fold in rhubarb. Spoon and spread batter evenly in sprayed pan.
Bake at 350°F. for 25 to 35 minutes or until cake springs back when touched lightly in center.
In small bowl, combine all glaze ingredients; beat until smooth. Spread glaze evenly over top of hot cake. Cool 10 minutes. Serve warm.