• Preheat oven to 180°C (350°F) conventional (see note 1 if using a convection oven with a fan) and generously grease a 9x13-inch cake pan (I use my homemade cake release). Set aside.

Crumb Topping

• In a medium bowl, whisk together the melted butter, brown sugar, granulated sugar, cinnamon, and salt until combined.
• Add the flour and gently mix with a fork using a fluffing motion until the mixture is clumpy. Do not overmix or it will start to turn into a dough. Set aside.

Cake

• In a medium bowl, sift together the flour, cornstarch, baking powder, baking soda, and salt. Whisk to combine and set aside.
• In a large bowl, add the butter, oil, and sugar. Using a hand or stand mixer on medium speed, cream for 2 minutes until light and fluffy.
• Add the eggs one at a time, mixing on low speed and making sure each egg is fully incorporated before adding the next.
• Add the buttermilk and vanilla and mix on low speed until well combined. The batter may look slightly curdled at this stage, but that’s fine. Now set your mixer aside as the remainder of the batter will be finished by hand.
• Add the sifted dry ingredients to the wet mixture and gently fold with a spatula until just combined. Be careful not to overmix, or the cake will turn out dense instead of soft and fluffy.
• Pour the batter into the prepared pan and spread it out evenly. Sprinkle the crumb topping evenly over the batter, making sure to leave some larger chunks for texture.
• Bake for 38–40 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
• Let the cake cool in the pan for at least 30 minutes before slicing and serving. Run a thin knife around the edges of the pan to loosen the cake and make removing slices easier.