Whip the cream: Pour 1 liter of heavy cream into a blender, stand mixer with whisk attachment, or food processor. Mix on medium-high speed until stiff peaks form, about 3-4 minutes.

Continue mixing: Keep mixing past the whipped cream stage. At 5-7 minutes, the mixture will become grainy and turn more yellow in color.

Separate the butter: At 8-10 minutes, the butter will suddenly separate from the buttermilk, leaving you with a solid yellow mass and liquid in your bowl.

Strain and collect: Pour the mixture through a mesh strainer to separate the butter from the buttermilk. Reserve the buttermilk in the refrigerator for other uses.

Wash the butter: Place the butter in a bowl of ice water. Squeeze and knead the butter with your hands to remove remaining buttermilk, changing the water 2-3 times until it runs completely clear.

Season and shape: Add salt or desired flavorings to taste, then shape the butter into a rectangle or desired form.