Preheat oven to 375°F.

Line a muffin pan with 12 paper wrappers; set pan aside.

Using a large bowl, cream butter and sugar together until light and fluffy (~2-3 minutes on medium speed with an electric mixer).

Add eggs; mix on low speed until well combined.

Using a separate bowl, sift together all-purpose flour, cornstarch, baking powder and salt; stir until well combined.

Add half of the dry mixture to the bowl with the butter and sugar; mix on low speed until well combined.

Add milk and vanilla extract; mix on low speed until well combined.

Add remaining dry ingredients; mix on low speed until well combined.

Divide batter evenly into prepared pan.

Bake for 16-18 minutes, or until a toothpick inserted into center of cupcakes comes out mostly clean.

Let cupcakes cool in pan for 10 minutes before transferring to a baking rack to finish cooling.

Using a sharp knife, slice each vanilla bean lengthwise.

Using the back of the knife, scrap the seeds out of the halved vanilla beans. Add the seeds and butter to a large bowl.

Cream butter and seeds together on medium speed for 3-4 minutes, or until very smooth.

Add 5 cups of powdered sugar and 4 Tbsp of milk; beat on medium-high speed until light and fluffy. If needed, add either a bit more powdered sugar to thicken the frosting or a bit more milk to loosen it.

Using a piping bag (or ziptop bag with a corner snipped off) and a large round tip, frost the tops of the cooled cupcakes.

{Optional} If desired, sprinkle tops of cupcakes with colored decorations.