Get out a stand mixer and fit it with the whisk attachment. Alternately, you can use a large mixing bowl and a hand mixer (with a whisk attachment if you have one).

The heavy cream will expand as it's whipped, so make sure the bowl you're using can accommodate that.

Add the heavy cream to the bowl of the stand mixer (or mixing bowl to use with a hand mixer), and whisk on MED speed for about 10 minutes (the exact time may vary).

Once you start to see stiff peaks (here's a good guide if you're not sure what you're looking for:

), you can add the salt (if you're using it).

Keep whisking on MED speed. What you're looking for is for the butter and buttermilk to separate in the bowl. The butter will stick to the whisk, and the buttermilk will be the liquid in the bowl.

While you're waiting for that to happen; get out a smaller mixing bowl and set a mesh sieve over the top of it. Get out another bowl and fill it with cold water and ice cubes.

Carefully pour the contents of the stand mixer bowl over the mesh sieve or strainer, so the butter stays in the sieve and the buttermilk strains into the bowl underneath.

Pick up the butter and try to squeeze the remaining liquid out.

Dip the butter into the bowl with the ice water, then squeeze again to remove excess liquid.

Place the butter onto a piece of parchment or wax paper and form it into a log or a cube/rectangle (similar in shape to when you buy a box of 4 sticks of butter from the store).

Wrap the paper around the butter, and tie the ends with kitchen twine or any string/bread ties you have around. I like to go one extra step and add the wrapped butter to a resealable plastic bag as well.

Alternately, you can slice the butter into 4 equally sized sticks and wrap each stick individually.

Place the wrapped butter in the refrigerator, so it can firm up a bit more.

Butter should stay fresh for up to 3 weeks, or it can be frozen for up to 9 months.