Preheat the oven at 170°C.
Sieve and mix flour, corn starch, baking powder, and salt.
In a separate mixing bowl, mix and beat sugar and butter until white and fluffy. (around 2-3 mins)
Add beaten eggs little by little, mix well after each addition. (about 6-7 times)
Add milk little by little, mix well each time.
Fold in the dry ingredients. Don't over mix.
Scoop out ⅓ to another bowl, mix in the cocoa powder.
Put the mixture in the baking pan. Use a knife to cut through the batter and create the swirl. Don't overdo it.
Put in the oven at 170°C for 55 mins or when a toothpick comes out clean.