Preheat the oven to 180°C/fan 160°C/gas mark 4. Grease and line a 28x18cm tray bake tin.
Add the dates to a medium bowl and cover with hot water. Leave to soak for around 30 minutes, then drain and blitz until smooth in a food processor.
Put the cocoa powder in a medium bowl with 6 tbsp boiling water and mix until smooth. Add the cocoa and butter to the bowl of a standing mixer and beat until smooth. Add the date purée and beat together until combined. Add the eggs one by one, beating well after each addition. Combine the baking powder with the flour, then fold into the date mixture until just combined. Fold in the milk and the vanilla extract.
Pour the mixture into the tin. Spread evenly and bake in the preheated oven for 40-45 minutes, or until risen, firm to the touch and shrinking away from the sides of the tin. Leave to cool in the tin for 10 minutes, then remove and leave to cool completely on a wire rack.
Preheat the oven to 190°C/fan 170°C/gas mark 4. Pierce the sweet potato in several places, then place on a small tray. Bake in the oven for 45 minutes to 1 hour until soft all the way through. Remove from the oven and leave until just cool enough to handle.
Scrape the insides of the sweet potatoes into a medium saucepan. Add the milk and dark chocolate and, on a low heat, continuously mix until the chocolate is melted and the mixture is mostly smooth. Transfer the mixture to your food processor while still warm along with the orange zest and 1 tbsp juice. Process for a 1-2 minutes until completely smooth. Leave to cool until room temperature, then spread on to your cake.