Preheat your oven to 350 degrees F. Line a muffin tin with 12 cupcake liners. Set aside.
Stir together the flour, cocoa powder, baking powder, and salt in a medium mixing bowl. Set aside.
In a large mixing bowl, beat together the sugar, vegetable oil, egg, and vanilla until combined and smooth. Pour in the buttermilk and mix until combined.
Add half of the dry ingredients to the wet ingredients and mix until just combined. Add the rest of the dry ingredients and mix until smooth. Scrape down the side of the bowl as needed.
While whisking, slowly pour the boiling water into the batter. Then add the chopped Oreos and stir to combine.
Pour the cupcake batter into the prepared muffin tin cups, filling each one about 2/3 full.
Bake for 20-22 minutes or until a toothpick inserted in the center comes out clean.
Remove cupcakes from the oven and carefully remove them from the pan. Let the cupcakes cool completely on a wire rack.
Once the cupcakes are cooled, prepare the frosting. To a large mixing bowl, ass the butter and beat with a mixer on medium-high speed until smooth and creamy.
Turn the mixer to low and add the powdered sugar in 2 additions. Beat until smooth and fluffy after each addition. Scrape down the side of the bowl as needed.
Add the vanilla and heavy cream and beat until smooth. Then add the crushed Oreos and fold them into the frosting using a rubber spatula.
Transfer the frosting to a piping bag fitted with whatever tip you like. Frosting the cupcakes and garnish with a mini Oreo cookie on top if desired. Serve immediately or chill until ready to eay.
Notes
If you don't have buttermilk on hand, you can substitute sour milk. Mix together 3/4 cup milk with 2 teaspoons of white vinegar, and let it sit for 5 minutes before using. Be sure to let the cupcakes cool completely to room temperature. It might not take a full hour, but better safe than sorry. Warm cupcakes will melt the frosting.