Cook pasta according to packages directions. Drain, reserving 1 cup of pasta water. Set aside.
Heat olive oil in a large skillet over medium heat. Cook garlic and capers just until garlic becomes fragrant, about 30 seconds. Stir in prawns. Cook until opaque. 3 - 5 minutes. Stir in Dijon mustard and butter. Cook just until butter melts.
Add pasta to the pan. Toss to combine. If the pasta seems a bit dry, add a little of the reserved pasta water. Season to taste with salt and pepper. Sprinkle parsley over the top and serve.