Preheat the oven to 200°C. Roast the sweet potato on a baking sheet for 45 minutes to one hour, or until tender in the centre and slightly caramelised on the outside.
Meanwhile, toast the pecans on a baking sheet for five to minutes until lightly browned. Cool, then roughly chop.
Reduce the oven temperature to 180°C. Grease a 28 x 18-cm (11 x 7-in) brownie tin with a little rapeseed oil and line with baking parchment.
Let the potato cool until just warm, then peel and weigh out 200g of the flesh into a blender. Add the dates and blend until smooth, then transfer the mixture to a heatproof bowl with the chocolate and set over a pan of barely simmering water. Heat, stirring occasionally, until the chocolate is melted. Add the oil and tahini and stir until smooth. Cool slightly.
Whisk the eggs, coconut sugar and salt on medium-high speed for five minutes, or until paler and almost doubled in volume, then whisk in the chocolate mixture slowly until well blended. Fold through most of the pecans.
Scrape the mix into the tin, gently levelling off the top. Scatter the remaining pecans over the top. Sprinkle with a large pinch of sea salt, then bake for 22-25 minutes, or until puffed and looking dry on top.
Cool in the tin before slicing into squares, or resist the temptation to be tidy and cut into random pieces. Chill any leftovers in a covered container for up to three days.