1 & 1/2 teaspoons vanilla
1 teaspoon espresso powder
2 sticks salted butter, melted
1/2 cup dutch-process cocoa powder
4 eggs, room temperature
2 cups sugar
1/4 teaspoon kosher salt
1 & 1/2 cups unbleached, all-purpose flour
for the topping:
1 (10 oz.) bag mini marshmallows
1/2 stick salted butter
1/2 cup milk
1/2 teaspoon espresso powder
1/2 cup dutch-process cocoa powder
4 cups powdered sugar
Preheat the oven to 350. Lightly grease a 9 x 13" pan. (If making a cupcake, line one muffin tin with a cupcake liner.) Set aside.
In a small bowl, stir together the vanilla and espresso powder until combined. Pour the vanilla/espresso mixture into a large bowl and add the melted butter, cocoa, and eggs. Stir until well combined.
Stir in the sugar. Add in the salt and flour and stir until no lumps remain.
Pour the batter into the prepared pan (and cupcake liner, if using). Bake for 30-35 minutes, or until toothpick inserted in the center comes out clean. (Start checking the cupcake at about 18-20 minutes.)
Remove from the oven and place on a wire cooling rack. Immediately scatter the marshmallows over the warm cake. Set aside.