In a blender or mixer combine batter ingredients. Blend well, scraping sides occasionally.
In a large bowl, beat together filling ingredients and set aside.
Pour half batter into a greased 9x13 inch pan. Drop filling by spoonfulls over the batter. Spread evenly. If using fruit, add now. Pour remaining batter over top. Cover and refrigerate 2 hours or overnight.
Bake at 350 degrees for 50-65 minutes until puffed and light-golden brown. Serve with blueberry sauce.
In a small saucepan combine:
1 cup water.
1 cup granulated sugar.
2 tablespoons cornstarch.
1 cup blueberries.
Cook with medium heat on the stovetop until sauce thickens.