Drain the pineapple juice from the can of crushed pineapple (if this amount does not give you 3/4 cup, add additional pineapple juice) and bring pineapple juice to a boil in a small saucepan. Set aside the crushed pineapple to use later.
Remove pan from heat; stir in oats. Let stand 20 minutes.
Combine all-purpose flour, whole wheat flour, sugar, baking soda, vanilla extract, oil and egg in a large bowl; stir well.
Add oat mixture; mix well.
Spoon batter into an 8-inch square baking pan coated with cooking spray.
Bake for 25 minutes or until a wooden toothpick inserted in center comes out clean.
Let cool on a wire rack.
Beat cream cheese at high speed of an electric mixer until light and fluffy.
Add pineapple, and beat well.
Spread over cooked cake, and cut into squares.
Notes
Proper Mixing: Avoid overmixing the batter to keep the cake light and fluffy.
Check for Doneness: Use a toothpick inserted in the center; it should come out clean.
Cool Completely: Before frosting, ensure the cake is completely cool to prevent melting.
Frosting Consistency: Drain the crushed pineapple well for the perfect frosting consistency.