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1
Preheat the oven to 350.
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2
Line a 12-cup muffin tin with paper or foil liners.
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3
In a medium saucepan, melt the butter with the vegetable oil and water over low heat.
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4
In a large bowl, sift the flour with the sugar, cocoa powder, baking soda and salt.
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5
Add the melted butter mixture and beat with a handheld mixer at low speed until smooth.
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6
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.
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7
Pour the batter into the lined muffin tins, filling them about three-fourths full.
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8
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.
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9
Let the cupcakes cool slightly, then transfer them to a rack to cool completely.
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10
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.
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11
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.
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12
With the mixer at medium speed, carefully pour the hot sugar syrup into the egg whites.
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13
Continue beating the frosting until cool and billowy, about 5 minutes.
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14
Preheat the broiler.
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15
Mix the frosting with the graham crackers and mini marshmallows.
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16
Spread the frosting on the cooled cupcakes.
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17
Transfer the cupcakes to a baking sheet and broil for 30 seconds, or until the frosting is toasted; serve.
