In a medium saucepan, combine brown sugar, water, and salt, and bring to a boil over high heat. Reduce heat to medium low, and simmer, stirring occasionally, for approximately 20 minutes.
Remove from heat, and add rum and vanilla extract, whisking to combine. Gradually add enough confectioners’ sugar to make a pourable glaze.
Use immediately.
Recipe by TeaTime Magazine at https://teatimemagazine.com/butter-rum-cakes-recipe/