Weigh 25 grams of butter in a sealable jar. Add 1 liter of milk and close the jar. Leave the milk with butter at room temperature for 2 days. After 2 days, stir the butter starter and fill an ice cube tray. Freeze the butter starter.
Cultured Butter
Fill a canning jar with 1 to 2 ice cubes of butter starter and add 32 oz or 1 liter of cream. Close the jar tightly and leave it at room temperature for 4 days. Place the jar of cream in the refrigerator 2 hours before you're going to churn the butter.
Fill the mixing bowl, food processor, or churner with the fermented cream and mix until the butter and buttermilk separate. Use a cheesecloth or kitchen towel to strain out the buttermilk, then fill the bowl with cold water to wash the butter. Repeat this process until the water runs clear.
Squeeze out as much water as you can out of the butter, optionally add the salt, and shape it into the desired shape. Wrap the butter in wax paper, or parchment paper, or store it in a butter dish.
Notes
Just before the butter is formed, it seems that there is a soup with small lumps of butter. When you reach this stage, you only need to mix/churn for a little longer.
Don't throw away the buttermilk! Use it to make soda bread, make a marinade, add it to your baked goods, or drink it.