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 small saucepan, combine the granulated sugar with the water and bring to a boil over moderately high heat, stirring constantly until the sugar has dissolved, 2 to 3 minutes.
In a large bowl of a standing electric mixer, beat the egg whites with the pinches of cream of tartar and salt until soft peaks form.
With the mixer at medium speed, carefully pour the hot sugar syrup into the egg whites. Continue beating the frosting until cool and billowy, about 5 minutes.
Preheat the broiler. Mix the frosting with the graham crackers and mini marshmallows. Spread the frosting on the cooled cupcakes. Transfer the cupcakes to a baking sheet and broil for 30 seconds, or until the frosting is toasted; serve.