Prepare the pan:
Grease an 8″x4″ loaf pan with cooking spray. Line it with a large piece of parchment, folding the sides to fit snugly. Use clips to hold the parchment in place, then grease the parchment with cooking spray.
Whisk the mixture:
In a large bowl, whisk together the milk, sugar, heavy cream, cocoa powder, cornstarch, and salt until everything is well combined.
Simmer the mixture:
Strain the milk mixture into a medium saucepan. Place it over medium heat and bring it to a simmer, stirring constantly to prevent sticking. Simmer for 2 minutes.
Melt the chocolate:
Reduce the heat to medium-low and stir in the chocolate chips. Stir until the chocolate is fully melted and the mixture is smooth.
Pour and chill:
Remove the saucepan from heat. Pour the chocolate mixture through a fine mesh sieve into the prepared loaf pan. Refrigerate for at least 4 hours, or until set.
Unmold the cake:
Carefully unmold the pudding cake onto a plate. Peel off the parchment and dust the top with extra cocoa powder for a finished look.
Slice and serve:
To slice, heat a knife by dipping it into boiling water, wiping it clean, and then slicing the cake. Alternatively, use a kitchen torch to warm the knife before slicing. Serve the pudding cake cold.