Using the white and light green parts only, finely chop 2 leeks and rinse well to remove dirt. Slice the remaining leek thinly in half moons, rinse well and reserve. Melt 1 tablespoon butter in a medium sauté pan over medium high heat. When the butter begins to brown, add finely chopped leeks and sauté until wilted, not browned. Season with salt and pepper.
Add cream, cooking until it bubbles thickly. Cool and purée until smooth. Adjust seasoning to taste. Chill for 30 minutes. Mixture should be thickened. (Can be made in advance and refrigerated for 2 days.)
Preheat oven to 425°.
Bring a large pot of water to a boil. Salt well. Add the potatoes, bring to a boil, then reduce to a gentle simmer. Cook for 10 minutes, until just tender. Drain and lay out on paper towels to dry. Season both sides of the cod with salt and pepper.
Measure and cut 6 sheets of parchment paper (11 by 17 inches each) and fold each piece in half, so food can be placed on the center bottom half of each paper. Distributing the ingredients evenly among 6 portions, begin by spooning a tablespoon of leek cream, topped by potato slices, chorizo and reserved sliced leek. Dab each with 1 teaspoon soft butter and a sprinkle of salt. Top with a piece of cod. Sprinkle 1 tablespoon of corn and then a little parsley, drizzle with 1 teaspoon olive oil.
Fold top half of parchment over cod. Starting at one edge corner of the fold, turn the paper over to seal while overlapping the edges until you reach the other corner. Twist the end of the paper to lock. Place 6 papillote packets on a sheet pan and place into the oven. Bake for 20 minutes, until they are puffed up.
Remove from oven. With a large spatula transfer each papillote to a large warm dinner plate. Cut the paper open carefully to avoid very hot steam. Serve and enjoy!
Recipe by Chef Maria Helm Sinskey, Culinary Director, Robert Sinskey Vineyards, Napa, California