Combine the yellow cornmeal, salt and sugar in a large bowl. Add the boiling water and stir until combined. Set aside for 10 minutes.
Melt a few tablespoons of the butter on a cast iron griddle, cast iron pan or nonstick pan over medium heat. Add the batter by 1/4 cupfuls a few at a time, smooth the tops lightly, if necessary. Cook until golden brown on both sides and just cooked through, about 5 minutes per side. Repeat with remaining butter and batter. Yields about five 3-inch round cakes.
Stone Crab & Vinaigrette:
Combine the mustards, vinegar, tarragon, salt, pepper and a few tablespoons of water in a blender and blend until smooth. With the motor running, slowly add the extra virgin oil and blend until emulsified, season with honey to taste.
Heat the canola oil in a large saute pan over medium heat. Add the crab meat and cook just to heat through. Add the green onion, cilantro and salt and pepper and cook for 30 seconds longer. Divide the crab meat among the Johnnycakes and drizzle with some of the vinaigrette. Garnish with tarragon leaves.