Grind the pepper coarsely in a spice grinder or a pestle and mortar. Add the rest of the herbs, spices and sugar and mix well.
Peel the potatoes and cut into chunky chips approx 6cm (2½ in) long and 1.5cm (¾in) thick. Blanch in boiling water for 3 minutes until al dente.
Drain well and place in a large mixing bowl. Add the oil and season with sea salt. Scatter over 2 tbsp of the Cajun seasoning and mix thoroughly. (Store the rest in a spice jar for another time).
Barbecue the chips using a barbecue rack; lay out in a single layer and cook for 8-10 minutes or so turning a few times until lightly charred and cooked through. Alternatively, roast in a high oven, gas 6, 200°C, fan 180°C for 10-12 minutes.
Serve with a dipping sauce of sour cream mixed with some snipped chives and a sprinkling of paprika.
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