Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan with butter or cooking spray. Set aside.
In a mixing bowl, cream together the granulated sugar and softened butter until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, mixing well after each addition.
Combine the lemon zest and fresh lemon juice with the milk, then gradually add this mixture to the egg and butter mixture. Mix until well combined.
In another bowl, whisk together the flour, cornstarch, baking powder, and salt.
Slowly add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
Pour the batter into the prepared cake pan and spread it evenly.
Bake in the preheated oven for about 50 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow the cake to cool in the pan for 10 minutes before transferring it to a cooling rack to cool completely.
Once cooled, dust with powdered sugar if desired, slice, and serve.