Preheat Oven & Grease Pan: Preheat the oven to 356ºF (180ºC) fan-forced. Grease an 8x8 inch square baking pan with butter and line with parchment paper. Note: For non-stick pans, line the base with parchment paper and grease just the sides with butter and dust with flour for smooth cake sides. Set aside.
Mix Dry Ingredients: In a medium-sized mixing bowl, sift together the flour, cornstarch and baking powder. Add the salt and whisk until well combined. Set aside.
Cream: In a separate large mixing bowl, using either electric hand beaters or a stand mixer with the paddle attached, add softened butter and sugar to the bowl. Beat for 2 minutes until pale and creamy, scraping down the sides as needed.
Add Eggs: Add eggs one at a time, beating for 10 seconds after each addition, and scrape down the sides.
Add Liquids: Pour in milk and oil, beating on low-medium speed for 15 seconds until well combined.
Combine with Dry Ingredients: Add half of the dry flour mixture and beat until just combined, leaving some flour visible and not mixed in. Add the remaining flour. Beat on low speed until just incorporated, ensuring not to over-beat, just until the flour has combined then turn off the beater. If some excess flour is still visible around the sides of the bowl, gently fold it in with a spatula. Over-beating can make the cake tough and dense after baking.
Transfer Batter: Pour the batter into the prepared pan and smooth the top with a spatula or spoon.
Bake: Bake for 30—33 minutes, or until a skewer inserted into the center comes out clean, not wet batter.
Cool: Allow the cake to cool in the pan for 15 minutes, then carefully transfer it to a wire rack to cool completely before frosting.