Ingredients
Equipment
Method
Bake the cake
- Heat the oven to 350°F.
- Grease a 9x13-inch baking dish with the softened butter.
- Whisk the flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl.
- Add the eggs, buttermilk, vegetable oil, and vanilla extract, and whisk until smooth.
- Gradually stir in the boiling water; the batter will be thin.
- Pour the batter into the prepared dish and bake for 30 to 35 minutes, until a toothpick inserted in the center comes out with a few moist crumbs.
- Cool the cake in the dish for 10 minutes.
Fill the cake
- Use the handle of a wooden spoon to poke holes about 1 inch apart across the warm cake.
- Heat the milk and hot cocoa mix in a medium saucepan over medium heat, whisking until steaming and dissolved; do not let it boil.
- Remove the pan from the heat and add the chocolate chips.
- Let the mixture stand for 2 minutes, then whisk until smooth.
- Slowly pour the hot chocolate mixture evenly over the cake, aiming it into the holes.
- Let the cake stand for 30 minutes.
- Refrigerate the cake until completely chilled, at least 2 hours.
Top the cake
- Beat the cold heavy cream and powdered sugar in a large bowl until medium-stiff peaks form.
- Spread the whipped cream over the chilled cake.
- Scatter the mini marshmallows on top.
- Keep the cake refrigerated until serving.
- Refrigerate leftovers.
Notes
For neat slices, make the holes evenly spaced and pour the filling slowly so it reaches them. Store covered in the refrigerator for up to 3 days; freezing is not recommended because the whipped cream topping can lose its texture. For a dairy-free variation, use dairy-free butter, milk, cream, chocolate chips, and cocoa mix.
