Pour the cream into a food processor and process on high for about 5 minutes, or until the cream looks very curdled and it sounds splashy.
Strain the mixture through a fine-mesh sieve lined with cheesecloth and squeeze gently to remove as much of the buttermilk as possible.
Add the ice to a large bowl and cover with water. Add in the butter and, using your hands, squeeze and knead the butter just to get a bit more of the buttermilk out and lightly chill the butter. Drain away the water and ice, dry out the bowl and mash the salt into the butter.
Transfer to a serving vessel or wrap in wax or parchment paper and serve at room temperature or chilled.
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