Place about half of the diced strawberries and three tablespoons sugar in a small saucepan. Heat on very low heat, stirring occasionally, until strawberries soften up well and reduces to a thick syrup. Remove from heat and set aside.
Combine oatmeal, bisquick and cornstartch in a bowl (If you are using Splenda, you can add to this dry mixture). Set aside.
In mixing bowl, combine yogurt, eggs, sugar (If not using splenda), butter, and vanilla, mixing well after each addition
Add dry ingredients to wet, about a cup full at a time, scraping down sides of bowl. Mix on medium until well combined.
Gently fold in remaining strawberries and pour into 9 by 9 inch pan that has been sprayed with baking spray.
Pour strawberry syrup over batter. If it is too thick, just swirl it a bit on the batter and it will loosen up.
Bake in preheated oven at 350 degrees for approximately 45 to 50 minutes, until tester comes out clean. Cool 15 minutes in pan then turn out onto wire rack to completely cool. Dust with confectionery sugar, if desired.