Ingredients:
Cake
Cooking spray or butter, for greasing pan
280 grams(2 1/3 cups) all-purpose flour
198 grams(1 cup) granulated sugar
78 grams(1/4 cup plus 2 tablespoons) light brown sugar
100 grams(1 cup plus 3 tablespoons) cocoa powder
2 teaspoonsbaking powder
1 teaspoonbaking soda
1/2 teaspoontable salt
200 milliliters(3/4 cups plus 1 tablespoon) boiling water
1 tablespooninstant coffee
200 milliliters(3/4 cups plus 1 tablespoon) vegetable or sunflower oil
2 large eggs, at room temperature
1 1/2-2 teaspoonsrose water, to taste
100 milliliters(1/4 cup plus 3 tablespoons) buttermilk, at room temperature
200 grams(4 cups) mini marshmallows
Icing
100 grams(3/4 cup plus 2 tablespoons) shelled pistachios
70 grams(5 tablespoons) unsalted butter
6 tablespoonswhole milk
42 grams(1/2 cup) cocoa powder
200 grams(1 3/4 cups) confectioners’ sugar
Directions:
Heat the oven to 350°F/180°C. Grease a 9×13-inch cake tin and line with parchment paper with overhang on the two long sides.
Make the cake batter: In a large bowl, combine the flour, both sugars, cocoa powder, baking powder, and baking soda, and salt.
In another large heatproof bowl, combine the boiling water and instant coffee and let sit for 5 minutes. Whisk in the oil, eggs, rose water, and buttermilk. Pour the coffee mixture into the flour mixture and beat for 1 minutes until smooth. Pour the batter into the prepared tin. Bake for about 20 minutes, until a skewer inserted in the center comes out clean.
Sprinkle the marshmallows over the surface of the cake and return it to the oven for about 5 minutes until they have melted slightly (but don’t take on any color—this will vary by oven). Let the cake cool in the tin for 5 minutes before pulling it out by the parchment overhang and transferring to a wire rack to cool completely.
Make the topping. Use a food processor or knife to very finely chop the pistachios. In a medium saucepan over medium heat, whisk together the butter, milk, and cocoa powder until the butter has melted. Remove from the heat and slowly whisk in the confectioners’ sugar until you have a smooth icing. Dollop the icing over the cake and spread smoothly with an offset spatula, then sprinkle the pistachios on top. Serve at room temperature. Store leftovers in an airtight container at room temperature for 2 to 3 days.