Preheat the oven to 350ºF/177ºC and line a cupcake pan with 12-15 liners.

Place all of the dry ingredients into the bowl of a stand mixer and stir on low for 30 seconds to fully combine them.

Add the vegetable oil, eggs, vanilla, and buttermilk and mix on low until just combined.

With the mixer still on low, add the hot water in a slow stream, then turn the mixer to medium and beat until smooth, about 2 minutes. The batter will be very thin.

Pour the batter into the prepared cupcake liners (no more than ⅔ full) and bake for 15-18 minutes, until a wooden toothpick inserted comes out with just a few moist crumbs on it. Cool completely before frosting.

Place the Oreo cookies into a food processor and grind them into fine crumbs.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter on medium-high speed until it's creamy and pale in color, about 5 minutes.

Add the powdered sugar a few cups at a time, mixing on low speed and scraping down bowl and paddle in between intervals.

Keep the mixer on low speed and add the vanilla, milk, and salt. Mix until fully incorporated and uniform, 1-2 minutes.

Mix in the Oreo cookie crumbs on low speed until uniform and well-dispersed.

Once the black velvet cupcakes are completely cooled, frost them with the Oreo buttercream. To create the look pictured, place the Oreo buttercream into a piping bag fitted with Wilton Tip 1M and pipe a swirl onto each cupcake. Top with crushed Oreo cookies.