Grease your baking tin of choice, then line it with baking paper, if using. Preheat the oven to 350°F.
In the bowl of a stand mixer or using electric beaters, combine 1 cup softened butter and 1½ cups white sugar. Beat until light and creamy. Periodically scrape down the sides of the bowl with a spatula.
1 cup butter, 1½ cups white sugar
Adding the eggs one at a time, crack in the eggs while still mixing. Add in the vanilla and mix until fully beaten through.
3 large eggs, 4 teaspoons vanilla extract
In a separate bowl, sift together the 2⅔ cups of flour, the ⅓ cup cornstarch, ½ teaspoon alt, and 3 teaspoons of baking powder, and whisk it all evenly together.
2⅔ cups all purpose flour, ⅓ cup cornstarch, ½ teaspoon salt, 3 teaspoons baking powder
Using a spatula instead of the mixer, gently mix ⅓ of the dry ingredient mixture into the butter and eggs, as well as half of the milk. Fold until incorporated, then repeat until all ingredients are combined.
1⅓ cups whole milk
Pour the batter into your prepared baking tin, smooth it out, and place it in the oven for 45-50 minutes or until golden brown all over and a toothpick inserted in the middle comes out clean.
Remove the cake from the oven and let it cool for 20 minutes. Once cooled, poke holes all over, about 1 inch apart, using a chopstick or the end of a wooden spoon.
Mix the lemon jell-o packet with 1 cup of boiling water and stir until dissolved. Very slowly pour the jello over the entire cake, allowing it to seep into all the holes.
3 oz lemon Jello, 1 cup water