Preheat your oven to 350°F and line a cupcake pan with liners. This recipe makes 12 standard cupcakes.
In a medium bowl, whisk together the flour, cornstarch, baking powder, and salt until fully combined. Set this aside for now--these are your dry ingredients.
In an electric mixer fitted with the paddle attachment, cream the butter and sugar together by beating them on medium speed for 3-4 minutes. Then, add the vanilla and eggs, one at a time, beating until the mixture is creamy and smooth.
In a microwave safe bowl or liquid measuring cup, heat the buttermilk for 10-20 seconds to get it to room temperature or just warm to the touch.
On medium-low speed, beat in half of the dry ingredients, followed by all of the buttermilk, and then the remaining dry ingredients. Fold until the flour is just combined. Do not overmix.
Divide the batter evenly between the 12 muffin cups. Bake for 20-22 minutes, or until the cupcakes are set and a toothpick comes out clean. Let the cupcakes rest for 10 minutes in the pan before transferring them to wire racks to cool completely.