Cream the sugar into the butter until soft and fluffy.
200 g unsalted butter, 250 g light muscovado sugar
Beat in the eggs and add the vanilla bean paste.
2 medium eggs, 1 tsp vanilla bean paste
Sift the flour, cocoa powder and baking powder in a separate bowl.
300 g self-raising flour, 65 g cocoa powder, 1 tsp baking powder
Fold the flour mixture in bit by bit. I found at the end that I needed to use my hands to combine it together fully and form a dough.
Add the chopped chocolate to the dough and mix well.
100 g chopped chocolate
Line a large round silicone baking pan with baking paper and spoon the dough in. Press the dough down with your fingers to fill the pan.
Place the pan in your slow cooker (I used a large 5.7 litre slow cooker that fits my silicone pan) and add two small jugfuls of water around the silicone pan.
Place the lid on the slow cooker, with a tea towel tucked in under the lid to stop the condensation dripping on the cookie cake.
Turn the slow cooker on low and cook for 2 hours.
After the two hours, add the chocolates on top. The top should not yet be firm, so you can press the chocolates into the dough.
140 g chocolates for topping
Cook for a further 30 minutes on high.
Check that the dough has risen and seem firm around the edges. It won't feel firm in the middle but it will firm up as it cools, as cookies do.
Remove the silicone pan from the slow cooker and leave the cookie cake to cool completely. If needed (or if you just fancy it) you can add more chocolate on top before serving!