Combine flour, cornstarch, baking powder, baking soda, and salt in a large mixing bowl.
Sift the ingredients. Set it aside.Protip — Sifting removes any lumps or impurities from the flour and eases the mixing of ingredients. Combine sugar and oil in a bowl.
Whisk this mixture until they are thoroughly combined. It will take about 2-3 minutes to blend everything together.
Add yogurt and essence to this mixture.
Whisk until it forms a smooth and lump-free mixture.
Add buttermilk and whisk until combined.
Add your dry ingredients to the wet ingredients. Whisk until just combined.
Scrape the bottom and sides of the mixing bowl with a spatula to ensure everything is thoroughly blended.
Pour the batter into the greased pan.
Smoothen the top layer and tap the pan to release the trapped air.
Generously spread the streusel layer over this batter. Lightly press the streusel down on the batter.
Bake for 35-40 minutes or until the toothpick inserted comes out clean.
Cool the cake for about 30 mins before slicing. This ensures a clean cut and avoids the disintegration of the crumb layer.
Once cooled, slice a piece and enjoy!