Lightly grease the bowl of your slow cooker.
In a large bowl, combine the flour, baking powder, brown sugar, and salt.
Add the melted butter, milk, vanilla, and golden syrup. Stir until a smooth batter forms.
Spread the batter evenly into the base of the slow cooker.
To make the sauce, add the brown sugar, golden syrup, butter, and salt to a saucepan.
Carefully stir in the boiling water. Heat until the butter and sugar has melted and dissolved.
Slowly pour the sauce mixture over the back of a spoon onto the pudding batter.
Place a clean tea towel underneath the slow cooker lid to absorb excess moisture.
Cook on HIGH for 1–1½ hours, or until the pudding is set on top.
Serve warm, making sure to spoon some of the caramel sauce from the bottom over each serving.