Turn on the slow-cooker to high and add the butter
When the butter has melted, add the pudding rice, stirring to coat all the grains
Add about a ΒΌ of the milk and stir
Once the slow-cooker contents have warmed up again, add the cocoa powder and stir well to combine
Pour in the rest of the milk, followed by the cream stirring to mix in. Put the lid on and let the contents warm up again
Once warm, add the caster sugar, stirring to dissolve
Stir in the vanilla extract. Replace the lid
Cook for 3 hours. After the first hour, you could stir the pudding to make sure the consistancy it to your liking. If it's too thick, add a little more milk and replace the lid. Repeat after another hour and again, just before it's finished cooking