Preheat oven to 350 degrees F.
Combine flour, cornstartch, baking powder and salt.
Beat sugar, eggs and vanilla on medium speed until light and fluffy, about 2 to 3 minutes. Add in vegetable oil and butter, beat until combined well.
Alternate adding in dry mixture and milk. Start with a 1/3 of the dry mixture, then add 1/2 of the milk, continue until all ingredients are incorporated. You should start and end with the dry mixture. Do not over mix.
Fold in diced strawberries.
Fill 24 regular cupcakes liners that have been inserted into regular sized cupcake baking tin. Fill to a little more than 3/4 full. These cupcakes don't rise a lot. Bake for 15-18 minutes until a toothpick comes out clean. Remove from oven and cool completely on a cooling rack.