Special equipment:
Measuring cups, Measuring spoons, Digital kitchen scale, Half sheet pan, Aluminum foil, Small rubber spatula, 2-quart saucepan, Wooden spoon, Small fine-mesh sieve, 1-cup heat-proof liquid measuring cup, Small bowl, Digital instant read thermometer, Blender, Large fine-mesh sieve, 4-cup liquid measuring cup, Six 4-ounce silicone molds, small bowls, glass jars, or teacups, Small skillet, Paper towels, Plate, Gallon-sized zip-top bag, Rolling pin, Small spoon
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Preheat the oven to 425 degrees F and place a rack in the center position.
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Place the bones cut-side down on a half sheet pan. Cover with a generous layer of aluminum foil, then use your hands to mold the foil around the bones and sides of the pan. Place a second half sheet pan on top of the foil, then flip the whole apparatus over. Remove the sheet pan that is now on top to reveal the bones, cut-side up and nestled in foil.
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Bake until the marrow has just softened, 10 to 15 minutes. Remove from the oven and rest until cool enough to handle, 5 to 10 minutes.
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Scoop the marrow out of the bones and into a 2-quart saucepan. Place over medium-low heat and cook, breaking up large pieces of marrow with a wooden spoon, until all of the marrow has rendered, 10 to 15 minutes. Strain through a fine-mesh sieve set over a heat-proof jar or liquid measuring cup. Measure out 1/2 cup of the rendered marrow, reserving the remainder for another use, such as roasting potatoes. Clean the saucepan; you’ll need it again.
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In a small bowl, stir together the gelatin and water and set aside to bloom for 5 minutes.
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Meanwhile, add the half-and-half, sugar, and kosher salt to the saucepan and place over medium-low heat until the mixture reaches between 160 to 170 degrees F, 3 to 5 minutes. Transfer to a blender and add the gelatin. Blend on low speed until the gelatin is dissolved, about 30 seconds.
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Add 6 tablespoons of the reserved rendered marrow to the blender, saving the remaining 2 tablespoons for the topping. Blend on medium speed for 1 minute, then strain through a fine-mesh sieve into a large measuring cup or bowl.
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Divide the mixture between six 4-ounce silicone molds, small bowls, 4-ounce glass jars, or teacups. Cover and refrigerate until the panna cottas are set, at least 5 hours.
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For the topping: Heat the remaining 2 tablespoons marrow in a skillet over medium heat. When the marrow is shimmering, add the cornflakes and cook, stirring frequently, until they turn golden brown, 2 to 3 minutes. Remove from the heat and stir in the smoked salt. Transfer to a paper towel-lined plate and cool.
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Transfer the cornflakes to a gallon-size zip-top bag and seal. Use a rolling pin to crush the flakes to form small flakes. Set aside for serving.
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When ready to serve, remove the panna cottas from the fridge and uncover. Top with the crushed cornflakes and a cherry. If you’d like to turn the panna cotta out onto a plate, top with a small plate, flip over, then gently push on the bottom of the mold to dislodge. Top and serve.