Preheat the KitchenAid 21L Digital Countertop Oven to 350ºF/180ºC and prepare muffin pan with muffin liners.
In a bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In another bowl, whisk together sour cream/yogurt, milk, and vanilla extract. Set aside.
In a Kitchenaid stand mixer bowl, beat with the flat beater the butter and sugar at medium speed until smooth and creamy.
Add egg white to the butter mixture gradually.
With the mixer on low speed, add in half of the dry ingredients followed by the wet ingredients. Add the second half of the dry ingredients and mix until combined.
With a rubber spatula, gently fold in the chopped Oreo cookies and sprinkles.
Place cupcake liners in the muffin pan and place an Oreo cookie at each base. Pour the batter three-fourths up each liner.
Bake for 25 to 30 minutes or until a toothpick inserted into the center of the cakes come out clean.
Cool on a wire rack for 10 to 15 minutes before removing the cakes from their pans.
Make the vanilla buttercream. In a KitchenAid stand mixer bowl, beat the butter using the whisk accessory until smooth and creamy.
Gradually add the powdered sugar.
Add the vanilla extract and mix on low speed. Gradually add the milk, checking for consistency.
Beat at medium-high speed for 3 – 5 minutes, until buttercream is extremely smooth and fluffy.