Discard chili stems. Shake out and discard chili seeds. Put chilies in a bowl and pour 1 1/2 cups boiling water over them. Let stand until soft, 6 to 7 minutes.
Meanwhile, in an 8- to 10-inch frying pan over high heat, stir or shake pumpkin seeds until they smell lightly toasted, 2 to 3 minutes.
Put pumpkin seeds in a blender or food processor. Lift chilies from soaking liquid and add to blender. Add garlic, vinegar, tomatoes, and cilantro. Whirl, adding enough chili-soaking liquid (about 1/2 cup) to make pipián mixture thin enough to scoop but not drippy. Add salt to taste. Scrape pipián into a small bowl.
Stand romaine lettuce leaves, stem ends down, in closely fitting bowl and drop ice cubes among the leaves to keep them crisp. To eat, scoop leaves into pipián dressing.