Remove the inner aluminium pot of the slow cooker and place on the stove top. Add the butter, condensed milk, a pinch of salt and the cocoa and bring to a boil, stirring constantly with a wooden spoon. Reduce the heat to medium-low; cook for 3-5 mins, stirring constantly, until the mixture is thick and shiny and starts to pull away from the bottom.
Stir in the vanilla and vigorously mix again. Pour the mixture onto a buttered plate or a plate covered with parchment paper. Chill till firm.
In a small bowl, whisk together flour, baking powder, and salt.
In a large bowl, beat together butter, brown sugar, and sugar until light and fluffy. Add vanilla and eggs and beat until combined.
Add flour mixture to wet ingredients and beat until just combined. (The mixture can now be chilled for about half an hour to make it easier to handle before proceeding).
Cover the base of your pot with half the mix. Add the butterscotch mix and then make another layer on top with the rest of the dough. Set your slow cooker to low for 4 hour cooking then turn it up to high for the last 30 mins uncovered.
Slice into bars and serve warm with vanilla ice cream on the side.