Pour the custard and milk into a medium saucepan and add the vanilla pod and seeds, cocoa powder, dark chocolate and sweetener.
Place over a medium-low heat and heat slowly, whisking gently until the chocolate is completely melted – this will only take a couple of minutes. Remove from the heat and discard the vanilla pod.
Add the chia seeds to the chocolate mixture and whisk again until well combined.
Divide half the raspberries between 4 small jars or glasses (about 280ml capacity). Spoon the chia mixture into the glasses and place in the fridge for 2 hours to chill.
Pile the remaining raspberries on top of the puddings to serve.