• Preheat oven to 180 °C (350°F) conventional (see note 1 if using a convection oven with a fan) and grease an 8x8 inch square cake tin (I use my homemade cake release). Line the bottom with parchment paper as well. Set aside.

Cinnamon Streusel

• Combine light brown sugar, all purpose flour and cinnamon. Mix until well combined (and no large chunks remain).
• Add in melted butter and mix until well combined. You should have a crumble like consistency. Place into the fridge while preparing the cake batter.

Cake Batter

• Sift together flour, cornstarch, baking powder, baking soda and salt. Using a whisk or fork, mix until well combined and set aside for now.
• In a large bowl combine butter, oil and white sugar. Using a hand or stand mixer on a medium speed, cream for two minutes until light and fluffy.
• Add in eggs one by one on a low speed, mixing well in between each addition.
• Add in vanilla and buttermilk and mix on a low speed until well combined. The batter may look a little curdled. Remove any batter off the attachments as the rest of the mixing will be done by hand.
• Add in presifted dry ingredients from earlier and fold with a spatula until just combined. Avoid overmixing - only mix until you can't see anymore large lumps or streaks of unmixed flour.
• Once the batter is done, remove the streusel from the fridge. It should have solidified a bit by now. Break the streusel up into small pieces (I use a spoon first, then finish with my hands). It should be a thick sand consistency (not quite as fine as regular sand) with small lumps throughout the streusel.
• Pour half of the cake batter into the cake tin and evenly spread it out with a spatula. Sprinkle half of the streusel mixture evenly over the top of the batter. Don't leave any gaps, so try and cover the cake batter as best you can. Once done, evenly spread the remaining batter over the top. The best way to do this is to place spoonfuls of the batter around the centre of the cake tin, making sure each spoonful of batter is touching eachother with no gaps. Once all the batter is in, begin spreading out the batter from the centre to the edges of the cake tin, without the spatula ever touching the streusal (see video for demonstration). Don't worry if its not perfect - it'll still taste incredible :)
• Finish by scattering the remaining streusel mixture evenly over the top of the cake. Try not to leave any gaps.
• Drop the cake tin very lightly on the counter once to remove any large air bubbles. Bake for 35 minutes, or until a toothpick inserted into the centre comes out clean or with a few moist crumbs on it.
• Allow the cake to cool in the cake tin for 20 minutes, before turning it out onto a wire rack and then turning it out again onto your serving plate (so that the cake is upright - see video for demonstration).

Vanilla Glaze

• While the cake continues to cool, prepare the vanilla glaze. Combine powdered sugar, vanilla and milk. Mix until smooth.
• Once the cake is slightly warm to the touch (you don't need to wait for the cake to completely cool), drizzle the vanilla glaze over the top. Slice and enjoy!