Preheat the oven to 365F degrees. Line 2 regular sized muffin tins with muffin papers. You'll end up with about 18 cupcakes total.
In a large bowl whisk together the flour, cocoa, cornstarch, baking soda, baking powder and salt until evenly combined (about 20 seconds). Then in a separate large bowl using a stand or hand held electric mixer, beat together the oil and sugars until no lumps remain. Add in the eggs and vanilla and continue beating for about 1 minute until well combined.
In a small bowl or liquid measuring cup, stir together the sour cream and buttermilk. Carefully beat about ½ of the sour cream & milk mixture into your oil & sugar mixture with the mixer on low speed. Then beat in the flour mixture, followed by the rest of the milk & sour cream. Beat until no lumps remain, but be careful not to overmix your batter.
Carefully spoon the batter into your prepared muffin tins, filling each muffin cavity about ½ to ⅔ full. Be careful not to fill your cupcakes too full. I like to use a large cookie scoop to make this easier. Bake for 15-17 minutes or until an inserted toothpick comes out clean. Allow the cupcakes to cool in their pan for 10 minutes then continue cooling on a wire rack.
While the cupcakes are cooling, make the peppermint frosting. In a large bowl using a stand or hand-held electric mixer beat the butter until well softened. Add in the powdered sugar about 1 cup at a time until you've added in 3 cups total. Then slowly add in the whipping cream about 1 tablespoon at a time while continuing to beat the mixture. Add in more icing sugar ½ cup at a time until you reach the desired sweetness level and consistency. Finally, beat in the peppermint extract and red food coloring, one drop at a time.