Grease a rectangular baking tin or small roasting tin, approximately 10"x10", then line with baking paper. Preheat the oven to 350°F.
Beat together 1 cup softened butter and 1 ½ cups white sugar until light and creamy. Don’t skimp on the beating time! Scrape down the bowl from time to time.
Add 3 large eggs one at a time and beat well between each.
Add 4 tsp. vanilla and beat through.
In a separate bowl, sift together 2 ⅔ cups all-purpose flour, ⅓ cup cornstarch, ½ tsp. salt, and 3 tsp. baking powder.
Using a spatula, gently mix ⅓ of the flour mixture into the butter and eggs, then half of 1 ⅓ cups milk, and repeat this process until all incorporated. Make sure not to over-mix at this point, the cake won’t be light and fluffy if you do.
Pour the batter into the baking tin, smooth out, and place in the oven for 45-50 minutes until golden all over and a toothpick inserted comes out clean.
Allow the cake to cool for 20 minutes, then poke holes all over about 1 inch apart using a chopstick or knife.
Mix 1 3 oz. pack of lemon jello and 1 cup boiling water and stir until dissolved. Pour the jello over the entire cake slowly, allowing it to seep into all the holes.