Preheat the oven to 350°F (175°C). Line a 12-cup muffin pan with cupcake liners. Optionally, place one whole Oreo cookie in the bottom of each liner for added texture and visual appeal.

In a large mixing bowl, sift together the all-purpose flour, baking powder, baking soda, and salt. Set aside.

In another large bowl, beat the softened butter and granulated sugar until the mixture becomes light and fluffy. Scrape the sides of the bowl as needed to ensure even blending. Add the whole egg and vanilla extract, and continue mixing until smooth.

Gradually add the buttermilk to the butter mixture. Follow by adding the sifted dry ingredients. Stir gently until just combined to avoid overmixing.

In a separate clean bowl, beat the egg whites using an electric mixer until stiff peaks form. Add approximately one-quarter of the whipped egg whites to the batter and mix thoroughly to lighten the mixture. Gently fold in the remaining egg whites until fully incorporated.

Fold the roughly chopped Oreo cookies into the batter until evenly distributed.

Divide the batter evenly among the prepared cupcake liners, filling each about two-thirds full. Bake for 18 to 20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Allow the cupcakes to cool completely before frosting.

In a large bowl, beat the softened butter until light and smooth, approximately 4 to 5 minutes. Scrape the sides and bottom of the bowl to ensure even consistency. Gradually add the powdered sugar, about ½ cup at a time, mixing thoroughly between additions.

Mix in the vanilla extract and gradually add milk or heavy cream until the frosting reaches a thick, pipeable consistency. Finally, fold in the finely crushed Oreos until evenly dispersed throughout the frosting.

Transfer the prepared frosting into a piping bag fitted with a decorative tip. Frost each cooled cupcake as desired. Garnish each with a whole Oreo or mini Oreo for presentation.