Grease the slow cooker and line with baking paper. Melt the coconut oil and dark chocolate in a heatproof bowl set over a saucepan of barely simmering water.
Once melted, stir in the cacao powder, maple syrup, agave syrup, vanilla, eggs and salt. Fold in the flour and baking powder until just combined, then stir in the walnuts and milk chocolate.
Pour into the slow cooker (see <