Line 15 cupcake holders with paper liners. Preheat the oven to 350 degrees Fahrenheit.
Place the chocolate and butter in a large microwave-safe bowl. Heat in the microwave on high for about one minute, until the butter is melted and the chocolate is just melted. Give it a good stir to mix it all together.
Stir in the granulated sugar, then add the eggs, stirring in one at a time. Then stir in the vanilla.
Add the flour and stir well. The batter should be smooth and light-ish brown.
Add a tablespoon of batter to each muffin cup.
Place one Oreo cookie flat on top of each cup of batter and press down slightly.
Top with the remaining batter, dividing between the 15 cups, about 1.5 tablespoons per cupcake.
Bake at 350 degrees until the brownies are set but not overbaked, 22-25 minutes. Cool completely.
Add the Oreo cookies to a zipper bag and seal. Crush with a rolling pin until they’re small crumbs.
In a stand mixer with the paddle attachment, beat the butter until smooth. Add the powdered sugar, Oreo crumbs, vanilla, and salt. Start out on the lowest setting and then increase to medium once everything is well-incorporated. Mix until completely blended. Drizzle in the heavy whipping cream and mix until fluffy, 1-2 more minutes.
Top cool cupcakes with the Oreo buttercream. You can use a pastry bag with a big frosting tip, or if you can’t find yours, a Ziploc bag works in a pinch. Just fill a big bag with the frosting, cut the corner off and squeeze the frosting out the corner onto the cupcake.
Top each cupcake with a mini Oreo.