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1
Submerge the baccalà in a large bowl of cold water. Refrigerate for 48 to 72 hours, changing the water every 8 to 12 hours. The cod is sufficiently desalted when it is pliable and a small piece tasted raw registers as mildly salty rather than sharp.
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2
Place the desalted cod in a wide saucepan. Cover generously with cold water, add the bay leaves, and bring to a bare simmer over medium heat. Poach gently — never at a rolling boil — until the fish just begins to flake when pressed with a finger and separates cleanly from the bone, 10 to 15 minutes. Reserve 100 ml of the poaching liquid. Remove the fish from the water.
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3
While the cod is still warm, remove all skin and bones with care. Transfer the warm flaked fish to the bowl of a stand mixer fitted with the paddle attachment, or to a large mixing bowl if working by hand. The fish must be warm at this stage: warm fish is the emulsifier, and cold fish cannot form a stable emulsion.
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4
This is where the dish lives or dies. Beat the warm fish on medium speed and begin adding the olive oil in a thin, continuous stream — precisely as one makes mayonnaise. The oil must enter slowly and steadily; added too fast, the emulsion breaks and the result is oily shredded fish rather than a cream. Continue beating and streaming oil for 5 to 10 minutes. Add a little of the reserved warm poaching liquid as needed to loosen the texture. The finished mantecato should be white-ivory, light, and spread like butter.
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5
Season with white pepper, a small squeeze of lemon juice, and salt only if required. The mousse should be mildly salty with a clean, light fish flavour and a subtle fruitiness from the oil. Adjust with further poaching liquid if the texture is too stiff.
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6
Grill or toast the white polenta squares until the cut faces carry distinct char marks and the interior remains soft. Spoon or pipe the mantecato onto each square at room temperature. Finish with a thin drizzle of olive oil and a single parsley leaf per piece.