To begin, take a 7-inch square pan and line it completely with non-stick baking paper, ensuring that the paper is higher than the pan.
Next, beat the eggs lightly and add Milo and condensed milk to the mixture. Whisk everything together until fully combined.
Pour the egg mixture into a non-stick pot or pan, and add the butter. Cook the mixture over medium-low heat while stirring continuously until it begins to thicken. Then, reduce the heat to low.
When the egg custard is fully cooked, it will appear thick and grainy. Remove the pot from the heat.
To assemble the cake, spread a single layer of custard into the pan, and arrange the marie biscuits on top of the custard. Repeat this step until all of the biscuits or custard have been used, ensuring that each interlayer has only a thin layer of custard.
After layering the cake, fold the high collars of the baking paper over the top of the cake, pressing down and compacting the cake with your hands.
Finally, chill the cake in the fridge overnight before cutting it into slices.