Grease your slow cooker pot using a little melted butter or oil.
In a large bowl, measure out the flour (250 g), caster sugar (140 g), cocoa powder (50 g), baking powder (1 tsp), orange zest and a pinch of salt.
In a jug, whisk together the orange juice, eggs (3), melted butter (100 g) and milk (150 ml).
Pour the ingredients in the jug over the dry ingredients in the bowl and mix until smooth.
Add in the chocolate pieces then pour the cake mixture into the slow cooker pot.
Mix 300 ml of boiling water with the light muscovado sugar (200 g) and 25 g cocoa powder for the sauce, then pour this over the top of the cake mixture.
Turn on the slow cooker, put on the lid (with tea towel under it to catch drips, stopping them falling onto the sponge as it cooks), then cook on high for 2-3 hours until firm.