Preheat your oven to 160ºC (320ºF) and line a 12-cup muffin tray with cupcake liners.

Combine the dry ingredients: flour, sugar, cocoa powder, espresso powder, baking powder, baking soda, and salt in a large mixing bowl. Whisk until well combined.

In another bowl, whisk together the wet ingredients: sour cream, vegetable oil, eggs, vanilla extract, and boiling water until smooth.

Slowly pour the wet ingredients into the dry ingredients, stirring gently until just combined. Aim for a smooth batter with no lumps.

Evenly divide the batter into the prepared cupcake liners, filling each about two-thirds full. Bake for 20-23 minutes or until a toothpick comes out clean.

Remove from oven and let cool in the tray for 5 minutes, then transfer to a wire rack to cool completely.

Crush the Oreo cookies for the buttercream and beat the cool butter in a mixing bowl until creamy.

Gradually add the powdered sugar, vanilla extract, milk, and crushed Oreos to the butter. Mix until smooth and fluffy.

Once the cupcakes are cooled, pipe the Oreo buttercream onto each cupcake and sprinkle with crushed Oreos if desired.