• Preheat the oven to 350F. Lightly grease a 8x8-inch baking dish with butter or non-stick cooking spray.
• Add the cake mix to a mixing bowl along with the egg and cream. Stir well until smooth and combined.
• Pour the batter into the prepared baking dish, then bake for 35 minutes or until risen and a toothpick inserted into the centre comes out clean.
• Allow the cake to cool.

For the chocolate filling:

• Add the condensed milk, chocolate chips and cream to a microwave-safe bowl. Microwave in 30 second bursts, stirring between each one, until it’s all warm and the chocolate has melted. Stir until smooth. Allow to cool slightly, then transfer to a piping bag with the tip cut off or a large nozzle inserted.
• When the cake is cool, pierce lots of holes in it with a straw.
• Squeeze the chocolate filling into the holes, then spread the top of the cake with most of the remaining chocolate filling. Set the piping bag aside for decorating the cake later.
• Place the cake into the fridge for 30 minutes for the filling to firm slightly.

For the marshmallow topping:

• Add the cream, powdered sugar and vanilla to a large bowl and beat with an electric mix until firm.
• Add the mini marshmallows (or chopped large marshmallows) to a microwave-safe bowl and microwave in 30 second bursts, stirring between each burst, until melted.
• Tip: Instead of melting marshmallows, you can use 1 cup of marshmallow fluff.
• Gently fold the slightly cooled marshmallow into the cream topping.

Assemble:

• Spread the marshmallow frosting onto the cooled cake. Add dots of the chocolate filling, from the piping bag, and use a toothpick to swirl it through the marshmallow.