Preheat oven to 350ºF degrees and line a cupcake pan with cupcake liners. Place an Oreo into the bottom of each liner. Add the espresso powder to the hot water, and stir until fully dissolved; set aside to cool.

In a large mixing bowl, whisk together the flour, cocoa powder, baking soda, baking powder, salt, crushed Oreos, and sugar until well combined. Add the oil, sour cream, and espresso liquid and continue stirring with a whisk. Stir in the egg, followed by the vanilla.

Fill cupcake liners about 3/4 full and bake in the oven for 20-22 minutes, or until a toothpick inserted in the center comes out clean. Allow cupcakes to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and salt together until smooth and creamy. Add the vanilla and 2 cups of the powdered sugar and beat until smooth.

Add 4 Tablespoons of milk, along with the remaining 2 cups of powdered sugar, and beat until combined. Add more milk, if needed to get the right consistency.

Frost cupcakes with the buttercream and decorate with crushed Oreos.