Make the Cupcakes: Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners. In one bowl, whisk together the flour, cocoa powder, baking soda, and salt. In another bowl, beat the softened butter and sugar until fluffy. Add the egg and vanilla extract next. Gradually alternate adding the dry ingredients mix with the buttermilk, blending until combined. Finally, stir in the hot water or coffee until the batter is smooth.
Fill each cupcake liner about 2/3 full with the batter and bake in the preheated oven for 18-20 minutes. Once done, allow them to cool completely.
Make the Cookies & Cream Filling: In a mixing bowl, beat the heavy cream and powdered sugar until soft peaks form. Fold in the crushed Oreo cookies and set aside.
Make the Frosting: In a separate bowl, beat the softened butter until fluffy. Gradually add in powdered sugar. Then, add the heavy cream, vanilla extract, and crushed Oreos. Mix until well combined.
Assemble the Cupcakes: Cut out the center of each cooled cupcake and fill with the cookies & cream filling. Pipe the Oreo frosting on top, drizzle with chocolate sauce, and garnish with a whole Oreo cookie.