4 tablespoons unsalted butter
1/4 cup vegetable oil
1/2 cup water
1 cup all-purpose flour
1 cup granulated sugar
1/4 cup plus 2 tablespoons unsweetened cocoa powder (not Dutch process)
3/4 teaspoon baking soda
1/8 teaspoon salt
1 large egg
1/4 cup buttermilk
1 teaspoon pure vanilla extract
4 ounces bittersweet chocolate, finely chopped
1/2 cup heavy cream
3 tablespoons unsalted butter
1 tablespoon light corn syrup
Pinch of salt
Chopped salted peanuts, for topping
Mini chocolate chips, for topping
Mini marshmallows, for topping
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 20 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.
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, about 15 minutes.
Place the frosting into a stand mixer with a whisk attachment and whisk on high for about 5 -7 minutes, until frosting is light and fluffy.
Mix the frosting with the peanuts, chocolate chips and marshmallows. Frost the cooled cupcakes and serve.