Step 1

Preheat your oven to 180°C (350°F) conventional (see note 2 if using a convection oven with a fan) and grease the bottom and sides of a 9x13-inch cake pan (I use my homemade cake release). Don't line the bottom, as the cake is served straight from the pan.

Step 2

In a bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Using a whisk, mix until well combined. Set aside.

Step 3

In a large mixing bowl, add the brown sugar, white sugar, melted butter, oil, milk, eggs, and vanilla extract. Using a whisk, mix until well combined.

Step 4

Add the sifted dry ingredients to the wet ingredients and whisk until just combined. Do not overmix.

Step 5

In a separate bowl or jug, mix instant coffee powder with hot water to make a strong coffee (or use espresso). Add the hot coffee to the batter and whisk again until just combined.

Step 6

Pour the batter into the prepared cake pan. Tap the pan lightly on the counter to release any large air bubbles. Bake for about 27 minutes, or until a toothpick inserted into the centre comes out clean or with a few moist crumbs.

Step 7

Once baked, immediately top the cake with the mini marshmallows. Return the pan to the oven for 2–3 minutes, or until the marshmallows are slightly melted and puffed up. Don’t overbake, otherwise the marshmallows will become tough.

Step 8

Once done, set aside to cool completely. Once cool, prepare the chocolate frosting.

Step 9

In a saucepan over low to medium heat, add the heavy cream, semi-sweet chocolate, unsalted butter, powdered sugar, instant coffee powder, salt, cocoa powder, and vanilla extract. Stir everything together, and let the mixture melt gently. Once smooth and silky, remove the saucepan from the heat. Do not let it simmer or boil.

Step 10

I like to run my frosting through a sieve to ensure there are no lumps, but this step is optional. Once the frosting is done, spoon the warm frosting evenly over the top of the cake. It helps to use a large spoon to distribute it, as spreading can be tricky over the marshmallows.

Step 11

Let the frosting set at room temperature for 30 minutes to 1 hour before slicing and serving.