Mysuostur is a soft brown cheese made from whey (mysa), which is a side product of cheese-making. It is boiled down until it thickens and caramelizes, just like dulce de leche, except it is not as sweet. Cheeses of this kind are made all over Scandinavia. Icelanders use the whey that is produced when skyr is made.
The soft, spreadable version is sold as Mysingur in Icelandic supermarkets. In Norway it is known as prim or myseprim.
5,75 liters (5750 ml) whey (if you can't get skyr-whey, use cheese whey)
1/2 tsp salt
1/4 cup brown sugar
2 tbs butter
2 tbs cream
Put the whey and salt into a wide-mouthed pan and cook, uncovered, until it has reduced by 2/3.
Stir in the sugar, butter and cream. At full boil, stir constantly until the cheese thickens. Test by putting a little on a plate. If it begins to set, the cheese is ready. Remove from the heat and stir until cooled. Store in jars…