Bring water to a boil in a small saucepan. Add couscous, reduce heat to maintain a gentle simmer, cover and cook until the water is absorbed, 8 to 10 minutes. Transfer to a fine-mesh sieve and rinse with cold water. Drain well.
Spread arugula on a serving platter. Add the couscous, corn, tomatoes, avocado, pepitas and currants in decorative lines over the arugula.
Combine basil, buttermilk, mayonnaise, lemon juice, garlic, salt and pepper in a mini food processor or blender; pulse until smooth. Top the salad with the dressing just before serving.
