Preheat your oven to 350 degrees and grease one 8 inch square cake pan. In a large bowl, beat together the sugar, softened butter, and canola oil until well combined. Mix in the egg and then add in the vanilla extract until just combined.

In a medium bowl, stir together the flour, salt, baking powder, and baking soda. Add about 1/3 of the mixture into the batter, and beat until just combined. Then add in about half of the buttermilk, again mixing until just combined. Repeat this process until all of the buttermilk has been added, and you have about 1/3 left of your dry ingredients. Add in the remaining dry ingredients along with the mini chocolate chips, mixing until well combined. You should have a fairly thick, smooth batter. Spread about half of the batter evenly into your prepared cake pan.

In a large bowl, stir together the sugar, brown sugar, cinnamon, salt, cornstarch, flour, and mini chips. Use a fork to stir in the melted butter and work the mixture together until a soft crumble forms. Sprinkle about 1/3 of the mixture onto the first half of your batter- it should be a complete layer covering the cake batter. Then gently spread the remaining cake batter on top evenly. Sprinkle the crumble evenly over the top of your prepared cake batter. Bake for 45-50 minutes, or until a toothpick comes out clean. Cool completely, slice, and enjoy!