Step 1 Oyster juice is used for the layer of jelly covering the "panna cotta." To make it, shuck the oysters and carefully reserve any juices from them. Strain the juice through a fine mesh strainer into a small bowl and add the oysters. Pour 1/4 cup of water over the oysters and refrigerate them, covered, for several hours or overnight.
Step 2 Cut the florets of cauliflower vertically through the stems into 1/2-inch slices. Spread the cauliflower evenly in a 3 1/2-quart saucepan and add the butter and 1 1/2 cups of water or enough water to come just to the top of the vegetables (they should not be submerged). Simmer over medium heat until the liquid is reduced by about half and the vegetables are fairly tender, 20 to 25 minutes. Add the cream and simmer to reduce the cream and completely cook the cauliflower, another 30 minutes. The contents of the saucepan should measure 2 cups.
Step 3Blend the cauliflower and cooking liquid in a food processor until completely smooth. Strain through a fine mesh strainer or a chinois. There should be about 1 1/2 cups of the puree. Add salt to taste.
Step 4Sprinkle 1 teaspoon of gelatin over 1 tablespoon of cold water; stir and let soften, 1 to 2 minutes. Warm half the cauliflower puree over medium-low heat until it barely simmers. Stir in the softened gelatin until dissolved, about 2 minutes. Stir in the remaining puree.
Step 5Spoon about 2 tablespoons of the "panna cotta" into the bottom of each small serving bowl and refrigerate for at least an hour to set. This can be done several hours ahead.
Step 6Remove the oysters (discard them) and strain the juice. You will need 1/4 cup of oyster juice. Place the remaining 1/2 teaspoon of gelatin and 2 teaspoons of water in a small metal bowl set over a pan of hot water. Stir constantly to dissolve the gelatin. Remove the bowl from the heat and add the oyster juice. Stir again to be sure that the gelatin and juice are completely combined. Add about 3 grinds of pepper. Place the jelly in the refrigerator and stir occasionally until it has thickened to the viscosity of salad oil and the bits of pepper are suspended in the liquid, about 1 hour.
Step 7Coat the tops of the chilled "panna cotta" with 1 teaspoon of the jelly, rotating the bowls to ensure an even coating. Return to the refrigerator to set, 30 minutes.
Step 8Before serving, garnish the top of each "panna cotta" with a "quenelle" of caviar: Lightly drag the edge of a teaspoon across the caviar, collecting it in a smooth shape resembling a football. This may take some practice to get right. If it's too much trouble, no one will complain about a simple 1-teaspoon mound of caviar.