Step 1

Combine buttermilk, yogurt, oil, lemon juice, mustard, anchovy paste, garlic, 1/2 cup Parmesan and 1/4 teaspoon each salt and pepper in a blender; puree on high speed until smooth, about 1 minute. Set aside.

Step 2

Cook pasta according to package directions, omitting salt. Drain, reserving 1 cup cooking water.

Step 3

Combine the pasta, chicken, tomatoes, 1/4 cup of the reserved cooking water and the remaining 1/4 teaspoon each salt and pepper in a large bowl. Stir in the buttermilk dressing until thoroughly combined. Stir in additional cooking water as needed for a creamy consistency. Cover and chill for at least 30 minutes or up to 2 days.

Step 4

Just before serving, stir in lettuce; sprinkle with the remaining 1/4 cup Parmesan.