Preheat the oven to 350°F.
Grease and flour a 10-inch bundt pan. Set aside.
In a medium bowl, whisk together the flour, cornstarch, baking powder, and salt. Set aside.
In a large bowl, using an electric mixer, beat the butter until creamy.
Gradually add the sugar and beat until light and fluffy, about 3 minutes.
Beat in the eggs one at a time, making sure each egg is fully incorporated before adding the next.
Mix milk, lemon juice, and vanilla extract in a small bowl.
With the mixer on low speed, gradually add the dry ingredients, alternating with the milk mixture, beginning and ending with the dry ingredients.
Fold in the sour cream and lemon zest and mix until just combined.
Pour the batter into the prepared bundt pan and smooth the top with a spatula.
Bake for 45 - 50 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 15 minutes before transferring to a wire rack to cool completely.