Preheat the oven to 180°C fan / 350°F. Line a 25 x 20cm / 8 x 10″ brownie tin with baking parchment.
Place the sweet potatoes on a large baking tray; roast for 40–45 minutes until tender. Set aside to cool. Reduce the oven temperature to 160°C fan / 350°F.
Peel the sweet potatoes (the skins should come off easily) and put them in a food processor, along with the oat milk and dates; blitz until smooth.
In a large bowl, whisk together the flour, baking powder, coconut sugar, cocoa powder and 1 teaspoon of flaky sea salt. Add the sweet potato mixture, followed by the melted chocolate, olive oil, vanilla and chocolate chunks. Stir until smooth.
Transfer the mixture into the tin and level with the back of a spoon. Bake for 20–25 minutes until just set. Sprinkle over the remaining sea salt and set aside to cool completely.