If you happen to have some leftover cream, it can easily be transformed into homemade butter.

Only use real, natural cream without any additives.

fresh heavy/whipping cream (fat content about 36 - 40 %)
cold water
(salt)

Cream should be chilled at about 12 °C. Slightly sour cream yields larger amount of butter.

Butter can be made using either a blender or an electric mixer. Pour the cream in the blender or the mixer bowl. If using a mixer, use a cover over the bowl so that the liquid resulting during beating will not splash around.

Blend or beat the cream at maximum speed until it separates into butter (the yellow, granulated stuff) and buttermilk (the white liquid), see pictures below.

Beaten cream separated into butter and buttermilk		Rinsed butter

Pour out the buttermilk and leave the butter in the bowl. Pour some ice-cold water on the butter and blend/mix at maximum speed for a few seconds more. Then pour out the water and scoop out the butter.

Using a wide spatula, press out water from the butter as much as possible. You can salt the butter to your taste, or leave it unsalted. Transfer the butter in a suitable, clean dish, cover tightly and store in refrigerator. Use like commercially made butter.
400 millilitres of cream will yield about 125 grams of butter.

Recipe source: adapted from instructions of Täpp Lars Arnesson, SVT/UR-akademin: "Fäbodliv: Kärna smör", 1997.