In a medium bowl, blend together the light brown sugar, all-purpose flour, and cinnamon until they are evenly combined and free of any large lumps. Stir in the melted butter until the mixture takes on a crumbly texture. Refrigerate the streusel to firm up slightly while you prepare the cake batter.

Sift the all-purpose flour, cornstarch, baking powder, baking soda, and salt into a separate bowl. Mix the dry ingredients thoroughly with a whisk or fork, then set aside.

In a large mixing bowl, cream the softened butter, vegetable oil, and granulated sugar together using a hand or stand mixer set to medium speed. Beat for two minutes or until the mixture is light, fluffy, and pale in color.

Add the eggs one at a time, mixing on low speed between additions to ensure even incorporation. Follow with the vanilla extract and buttermilk, blending on low speed. Don’t worry if the mixture appears slightly curdled; this is normal.

Gently fold the dry ingredients into the wet mixture using a spatula, working just until the ingredients are combined. Avoid overmixing to maintain a tender crumb.

Remove the chilled streusel from the refrigerator. Use a spoon or your fingers to break it into small, sand-like clumps with a few lumps scattered throughout.

Pour half the cake batter into a greased and parchment-lined 8x8 inch square cake pan. Spread the batter evenly with a spatula. Sprinkle half the streusel mixture over the batter, ensuring it forms a complete layer with no exposed areas.

Carefully spread the remaining batter over the streusel. To do this, place spoonfuls of batter across the pan and smooth them out gently with a spatula, avoiding contact with the streusel layer underneath. Finish by sprinkling the remaining streusel evenly over the top.

Gently tap the pan on your countertop once to release any trapped air bubbles. Bake in a preheated oven at 180°C (350°F) for 35 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 20 minutes. Then, transfer it to a wire rack to cool further. Once cooled slightly, flip the cake onto a serving plate so the top faces upward.

Prepare the glaze by whisking together the powdered sugar, vanilla extract, and milk or cream until smooth.

When the cake is warm to the touch but not hot, drizzle the glaze generously over the top. Slice into portions and serve. Enjoy the rich, cinnamon-swirled layers with a hint of sweet vanilla glaze in every bite!