Place sugar in a thick-bottomed sauce pan, and heat it on very low heat for a few minutes, until it starts to melt.

When no visible crystals remain, add sour cream, and mix well.

Simmer on lowest setting for 20-30 minutes stirring every now and then, or until the caramel becomes thicker and a bit darker.

Add butter and vanilla, mix to combine well.

Transfer to a parchment lined mold (I use a bread pan for a single batch, and a square brownie pan for a double batch).

Cool in refrigerator until it is thick enough to cut. Slice with a pizza cutter or a sharp knife. You can wrap it in pieces of parchment paper, if you wish.

I store it in an air tight container in refrigerator but you can store it at room temperature too.