How to Make It
Preheat the oven to 350°. Line a 12-cup muffin tin with paper or foil liners.
In a medium saucepan, melt the butter with the vegetable oil and water over low heat.
In a large bowl, sift the flour with the sugar, cocoa powder, baking soda and salt. Add the melted butter mixture and beat with a handheld mixer at low speed until smooth. Add the egg and beat until incorporated, then add the buttermilk and vanilla and beat until smooth, scraping the bottom and side of the bowl. Pour the batter into the lined muffin tins, filling them about three-fourths full.
Bake the cupcakes in the center of the oven for about 25 minutes, until springy and a toothpick inserted in the center comes out clean. Let the cupcakes cool slightly, then transfer them to a rack to cool completely.
In a medium bowl, using a handheld electric mixer, beat the softened butter at medium speed until smooth. Add the confectioners' sugar, vanilla extract and salt and beat the mixture at low speed just until combined. Increase the mixer speed to medium and beat until smooth. Add the milk or heavy cream and beat until light and fluffy, about 2 minutes.
Spread the frosting on the cooled cupcakes, top with the crushed Oreos and serve.