You can substitute raspberries in this recipe, or a combination of berries depending on what's available.
Preheat oven to 350 degrees F / 180 degrees C. Grease a 9x13 inch / 23x33 cm baking dish. (You can line the bottom with parchment paper to help make the cake pieces easier to remove from the pan.)
Prepare streusel by stirring together the flour, brown sugar and cinnamon until well blended. Toss the butter in the flour mixture, then cut the butter into the flour using a pastry blender or two knives. When the butter has been broken down into small pieces, use your fingers to make fine crumbs by rubbing the flour and butter mixture between your fingers and thumb. Set the streusel aside while you prepare the cake.
In large mixing bowl, stir together the flour, cornstarch, sugar, salt, baking powder and baking soda. Add the eggs, oil, sour cream or buttermilk, and orange rind (if you are using it). You can beat the ingredients in a stand mixer, with a hand mixer or with a whisk. With a stand or hand mixer, mix on low until ingredients are just blended. Scrape sides of bowl, then beat on medium speed for 3 minutes. The batter will be thick. If you are mixing by hand, whisk the batter thoroughly until it is very thick, scraping down the sides of the bowl occasionally.
Spread half the cake batter into the prepared pan. Sprinkle half the blueberries over and cover with 1/3 of the streusel. Top with the remaining batter. Sprinkle remaining blueberries over the batter, and distribute the remaining streusel as evenly as possible over the cake.
Bake for 45-50 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Place a tray under the cake while it is cooking to prevent any drips.
Remove the cake from the oven and let cool for 10 minutes. Make the glaze by stirring together the icing sugar and cream or milk, and drizzle over cake. Serve warm or cooled.