Measure out the two sugars into a bowl. Measure out the butter and cut into cubes.
150 g butter, 75 g light muscovado sugar, 100 g caster sugar (superfine sugar)
Cream together the butter and sugar until light and fluffy. I use a wooden spoon and it takes around 5 minutes.
Add the eggs and vanilla extract and beat well.
2 eggs, 1 tbsp vanilla extract
In a different bowl stir together the flour, baking powder and salt.
340 g plain flour (all-purpose flour), 1/2 tsp baking powder, 1/4 tsp salt
Next stir the flour mixture into the butter mixture until it all comes together like cookie dough. The sides of the bowl should practically be clean when it is the right consistency.
Stir in the choc chips (and raisins/sultanas if using) and make sure they are evenly distributed through the cookie dough.
100 g dark choc chips, 50 g sultanas
Grease your slow cooker bowl with butter, cut baking paper to fit the pot (slits in the sides of the paper help to fit to the oval pot) and put it in. Grease the top of the paper too.
Spoon the dough into the slow cooker bowl and flatten/press into the sides.
Slow cook for two and a half hrs on low, with lid on. Switch to high for a further half an hour, leaving the lid half open.
Take the bowl out the slow cooker and cool for half an hour. Using the edges of the baking paper to pull it out, remove the cookie from the pot and put it on a cooling rack. Remove the grease proof paper from the bottom, and slice to serve.