In a small bowl, add the milk and vinegar and give it a whirl. Let sit a minute. In a large bowl, combine all the dry ingredients of the cake and whisk together. In a medium sized bowl, mix together the oil, milk, water/cornstarch mixture, and vanilla. Mix well.

Incorporate the wet ingredients to the dry ingredients, scraping the sides of the bowl with your spatula.

Reserve 1 ½ cups of batter. Spread remaining batter in the bottom of a 9×11 greased baking dish.

Spoon on the blueberry filling evenly over the top, without touching the edges.

Spoon the rest of the batter over the top in clumps, leaving some of the blueberry filling showing through (swirl the batter around gently).

Bake at 350 degrees F for about 40 minutes. Let cool at room temperature. If desired, top with a sprinkle of powdered sugar, or simple powdered sugar frosting. Cut into squares before serving. Store covered at room temperature.