Corn Soup with Harissa Yogurt

INGREDIENTS

• 6 ears corn, husks removed
• 7 cups / 1.65 liters water
• 2 tablespoons olive oil
• 1 tablespoon unsalted butter
• 1 pound potatoes, cut into 1/4-inch dice
• 4 medium shallots, chopped
• 3 medium cloves garlic, chopped
• 2 teaspoons fine grain sea salt
• freshly ground pepper
• plain yogurt, harissa, and fresh dill, to serve