Preheat oven to 350°F. Grease a 9- x 13-inch pan with one teaspoon of oil. Press a piece of parchment paper into the bottom and up the sides of the pan and grease again.

To make the batter, place the yolks, sugar, brown sugar, vanilla and water into a medium bowl and beat with an electric mixer for one minute at medium speed. Add the potato starch and beat for two minutes on medium speed until well-mixed. Set aside.

In a separate bowl, beat the egg whites on low speed until foamy, add the salt and then beat on high speed until stiff. Fold the whites into the batter in four parts, mixing increasingly slower with each addition.

To make the streusel, in another medium bowl, mix the cinnamon, brown sugar and chopped nuts. Add the margarine and mix (hands work best) until the mixture clumps together.

Spoon the batter into the pan. Scatter the blueberries on top. Bake for 20 minutes, then remove from the oven. Break apart the streusel and scatter the pieces over the top of the batter. Bake another 20 minutes or until a toothpick inserted comes out clean. Serve warm.