Preheat your oven to 180 degrees and grease a baking tray.
Chop the rhubarb into small pieces.
Allow the whole packet of Shetland butter to soften, then chop it into smaller pieces.
In a mixing bowl, combine the softened butter with 180 grams of caster sugar and beat them together using electric beaters.
Add the eggs one at a time, mixing well after each addition with the electric beaters.
Incorporate 250 grams of self-raising flour into the mixture and blend until everything is well combined.
Pour the thick batter into the prepared tray and smooth the surface.
Spread the entire tin of ready-made custard over the batter and use a knife to swirl it in gently.
Sprinkle the chopped rhubarb, which has been mixed with a couple of tablespoons of sugar, evenly on top.
Bake in the preheated oven for about one hour.
