Dissolve the custard powder in 1/2 cup milk in a small bowl and set aside. Then in a saucepan, stir the sugar into the remaining 2 and 1/2 cups milk and bring to a simmer on low heat.
Stir the peanut butter into the warm milk, till melted though I found it doesn't fully dissolve, you can see specks of peanut butter throughout which isn't a bad thing.
Into the saucepan, pour in the custard powder+milk mixture slowly and whisk constantly. In a few seconds the mixture will begin to thicken. Cook for about 3 to 4 minutes till it coats the back of a spoon and when you run your finger down the spoon, it should leave a trail.
Turn off the heat and let the custard cool for about 30 minutes, before transferring to the fridge till completely chilled and no longer runny. A skin will inevitably form on top so you can skim off the top just before serving. If it still seems runny, see notes on what to do.
While the custard chills, blitz the biscuits in a bowl of a food processor until crumbed, not super finely ground. You want there to still be bigger bits scattered throughout.
Now place the banana slices in a single layer on a baking tray lined with a silicone mat or non stick baking paper. The mat makes clean up the easiest because sugar will inevitably melt as the caramelisation happens.
Sprinkle the bananas liberally with the sugar, then place under the oven broiler for about 10 minutes until the bananas are caramelised and the sugar is deeply browned. Make sure you place the tray in the center of the oven, not very close to the rods as the sugar burns very fast. If you would like to do this on the stove, melt the sugar and then place the banana slices on it, letting them cook and caramelise. Allow the banana slices to cool completely.
To assemble the puddings, choose your favourite jars or glasses, 4 to 5 medium sized ones will work well, but there are no rules. You could also just assemble the pudding in a deep bowl.
Layer the biscuits, then custard, then bananas, repeating until all of it is used up. Reserve a spoonful of crumbs to decorate the top, and drizzle with more peanut butter. Chill for 20 minutes before digging in! Will keep in the refrigerator for 2 to 3 days, but the crumbs will soften quite a bit. Happy pudding!