Soak split peas in water (enough to cover by a couple inches) for at least 3 hours.
Slice sweet potato into matchsticks. Some mandolins have a convenient feature that speeds the process. Sauté the matchsticks over medium heat until soft and cooked, but not browned. Lightly sprinkle with salt and remove from heat and set aside to cool in mixing bowl.
Drain the split peas. Add peas, 1/3 cup of water, green peppercorns and optional ghee (if using) to a food processor and puree for 5 minutes. Scrape the sides of the bowl down after the first minute. The result should be fairly creamy with no chunks of pea.
Add the baking soda and salt and pulse for a few seconds until well mixed, about 5 seconds—don't overblend at this stage. Add the cilantro and pulse briefly so it is coarsely chopped.
Decant the pea batter into the bowl with the cooked sweet potatoes. Combine gently with a rubber spatula.
Cook the mixture in a frying pan on medium-low in about 2 teaspoons of oil or ghee. The ideal pan for this is an 10-inch nonstick skillet, which allows you to form a perfectly round pancake. Gently spread the batter into the pan, smoothing the top so it is even and using your spatula to bring in the edges so they're smooth and vertical. Place a lid over the skillet to allow the pancake to steam somewhat. The pancake is ready to turn when the bottom is golden brown and lightly crispy. Flip and cook the second side, uncovered, on medium until golden brown as well.