Make the cake: 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.
Meanwhile, make the frosting: Put the chopped chocolate in a heatproof medium bowl. In a small saucepan, heat the cream with the butter, corn syrup and salt until hot but not boiling. Immediately pour the mixture over the chocolate. Let stand for 5 minutes, then whisk until smooth. Let the frosting cool, whisking occasionally, until thick enough for dipping and glazing the cupcakes, about 30 minutes. For a frosting firm enough to spread, let stand for 1 hour.
Mix the frosting with the peanuts, chocolate chips and marshmallows. Frost the cooled cupcakes and serve.