Preheat your oven to 350°F (180°C). Grease a 9-inch baking dish thoroughly with butter or cooking spray. A ceramic or glass dish works well here because it retains heat and keeps the sauce warm longer.
In a large mixing bowl, whisk together the dry ingredients: flour, cocoa powder, Milo powder, and sugar until well combined. Make sure there are no lumps of cocoa powder hiding in the mixture.
Make a well in the center of the dry ingredients. Add the eggs, milk, melted butter, and vanilla extract. Mix gently until a smooth batter forms. Avoid overmixing, as this can make the cake tough.
Using a spatula, fold in the chocolate chips. Distribute them evenly throughout the batter so every bite has a chance of melted chocolate.
In a small saucepan, prepare the chocolate sauce. Heat the heavy cream until warm but not boiling. Remove from heat and whisk in the Milo powder and chocolate chips until smooth. The mixture will thicken slightly as it cools.
Pour the cake batter into the prepared baking dish. Spread it evenly with a spatula. Slowly pour the chocolate sauce over the top, letting it pool across the surface. Do not stir the sauce into the batter.
Bake for 25 to 30 minutes. The edges should look set while the center still appears slightly jiggly. The key to perfect MILO PUDDLE CAKE is not overbaking it. If you bake it too long, the sauce will absorb completely and you will lose the puddle effect.
Allow the cake to rest for 5 to 7 minutes after removing from the oven. This resting period helps set the structure while maintaining the gooey center.
Serve warm by scooping generous portions into bowls. The chocolate sauce will flow beautifully from the center of each serving.