Heat a barbecue grill to medium–high. Place the corn cobs directly on the grill and cook, turning regularly, until lightly charred all over. Remove from the heat and allow to cool slightly, then shave the kernels from the cob using a sharp knife.
Prepare the cucumber by slicing it lengthways into ribbons using a vegetable peeler or mandolin. Place the corn kernels in a bowl with the cucumber, mint, herbs, shallot and chilli flakes.
In a small bowl, whisk together the vinegar, olive oil, honey, salt and pepper. Pour over the salad, toss gently and set aside.
Combine the yoghurt, harissa, lemon juice and a pinch of salt until smooth. Taste and adjust the heat or acidity as needed. Set aside.
Prepare the scallops just before cooking. Pat them very dry with paper towel — this is essential for a good sear. Season lightly with salt and pepper.
Heat a heavy-based frying pan (cast iron or stainless steel) over medium–high heat until hot. Add the olive oil and butter. When the butter has melted and is foaming, add the scallops in a single layer, making sure they are not crowded. Cook without moving them for about 2 minutes, until a deep golden crust forms. Turn and cook for a further 1 minute, just until opaque through the centre. Remove from the pan immediately.
To serve, spoon the harissa yoghurt onto the base of each plate. Top with the barbecued corn and cucumber salad, then place the scallops on top or alongside. Finish with a squeeze of citrus, freshly cracked pepper and extra herbs if you like. Serve straight away.