Preheat the oven to 350-degrees.
First in a large bowl or a bowl of a stand mixer fitted with a paddle attachment, beat the butter for 2-3 minutes on medium speed until light and fluffy. Next, add the sugar and beat for another 2 minutes on medium speed.
After you have done that add the eggs and vanilla extract and beat once again for about 2-3 minutes. You want the batter to be light and fluffy. Then stop the mixer and add the sourdough starter discard. Beat this until combined, about 1 minute.
In a separate medium bowl combine the flour, cornstarch, salt and baking powder.
With the mixer on low, slowly add the dry ingredients to the wet and mix until just combined.
Remove the bowl from the mixer and give the batter a final stir with a rubber spatula to make sure it is all combined well.
Grease a half sheet cookie sheet (17" x 12") and spread the batter out evenly. The batter may be a little sticky. If you are having trouble getting it to spread with a spatula you can use a lightly floured hand to work it out evenly in the baking sheet.
Place the pan in the preheated 350-degree oven and bake for 19-22 minutes or until lightly golden brown on the edges.
Remove from the oven and cool before assembling the pizza.