Melt butter in a medium saucepan over medium heat. Add couscous and cook, stirring frequently, until lightly toasted, about 2 to 3 minutes.
Add broth, season with salt and pepper and bring to a boil. Reduce heat to a simmer, cover and cook until liquid is absorbed, about for 10 minutes. Remove from heat and let sit covered, for 5 minutes
Whisk together olive oil, vinegar, Dijon, garlic, lemon zest and juice. Season with salt and pepper. Add cherry tomatoes, yellow pepper, green onion, couscous, parsley, basil and chives and toss to combine.