Preheat your oven to 180°C (350°F). Grease and line a cake pan with parchment paper.
In a saucepan over medium heat, warm the milk until it’s just about to boil. Remove it from the heat and let it cool slightly.
In a large mixing bowl, whisk together the castor sugar and cornstarch.
Slowly add the warm milk to the sugar and cornstarch mixture, whisking continuously to avoid lumps.
Add the vegetable oil and slightly melted butter to the mixture, and continue to whisk until well combined.
Gradually add the dry ingredients (flour) to the wet ingredients, stirring until you have a smooth batter.
Pour the batter into the prepared cake pan.
Bake in the preheated oven for approximately 25-30 minutes or until a toothpick inserted into the center of the cake comes out clean.
Allow the cake to cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.
Once the cake is completely cooled, you can frost or decorate it as desired.
