In a large bowl, add the brown sugar, melted butter, eggs, vanilla extract, orange zest, and milk. Using an electric handheld mixer or whisk, mix until well combined.
Add the self-raising flour and cocoa powder, then mix until smooth.
Fold through the chopped dark chocolate and pitted cherries.
Butter the pot of your slow cooker and tip the batter in. Spread to form an even layer across the bottom.
Add the sauce ingredients to a small pot and heat until the sugar dissolves and the mixture is well combined.
Gently pour the sauce over the chocolate batter. Cover the slow cooker pot with a tea towel before putting on the lid. This will absorb any excess moisture.
Cook on high for 1 hour and 25 minutes to 2 hours. About 1 hour and 10 minutes into the cooking time, scatter over the grated milk chocolate if using.
To test if the pudding is done, the top should be springy to the touch, and the top two-thirds of the batter should be cooked through when tested with a sharp knife. There should still be molten sauce at the bottom.
Serve warm with vanilla ice cream.