In a large bowl: combine the eggs and butter then beat. Then add brown sugar, white sugar, vanilla extract, milk while continuing to beat until smooth. Add the flour, cornstarch, baking powder and salt then mix. Highly recommend that you sift the dry ingredients before mixing them into the wet ingredients. This will form a smoother batter and the cake will be very soft & fluffy.
Cinnamon Layer (filling)
In a separate bowl: combine all the filling ingredients together.
Streusel Layer (topping)
In a separate bowl: combine flour, brown sugar, cinnamon and COLD butter. Use a pastry cutter or your hand to mix until it begins to form coarse crumbs.
Bake
Preheat the oven to 350°F. Grease the 9" x 9" pan with baking cooking spray or butter. Recommend putting some parchment paper on the bottom of the pan first.
Add ½ of the batter to the pan. Add all of the cinnamon layer on top (use a spoon to spread/level). Place the other ½ of the batter on top. Coat the top batter layer with the streusel mixture (use a spoon to spread/level). Watch the video for reference.
Bake for 45-50 minutes (it took me 45 min) or until a toothpick placed in the center comes out clean. Remove from the oven and rest the cake in the pan for 15 minutes. Then transfer the cake to a baking rack. Cool for 15 minutes or to room temperature prior to serving.