Place the chile on a broiler pan and place the pan under the broiler until the chile is charred. Turn the chile over and char the other side.
Place the chile in a bowl, cover the bowl, and allow the chile to steam for about 10 minutes.
Remove stem, skin, and seeds from chile.
Toast pumpkin seeds: heat a small skillet over medium-low heat and add pumpkin seeds. Heat seeds until they are lightly toasted, shaking the pan occasionally to toast seeds on both sides.
Place chile, pumpkin seeds, garlic, cheese, vinegar, lime juice, oil, salt, and pepper in a food processor and blend until pieces of pumpkin seeds can no longer be seen. Add cilantro and blend until cilantro becomes small flecks.
Add mayonnaise and blend until combined.
Scrape into a bowl and whisk in 1 tablespoon of water. Add more water if you want the dressing thinner.
Taste and add salt if needed.
