In a small sauce pot combine the sweetened condensed milk, dulce de leche, dark chocolate, cocoa powder and salt. Place the pot over medium high heat and bring to a gentle boil, stirring constantly with a wooden spoon. Reduce heat to medium-low and cook 10 to 15 minutes, stirring constantly, until mixture is thick and shiny and starts to pull away from bottom and side of sauce pot. The mixture is going to get thick. Remove from the heat and stir in the butter, vanilla and rum until melted and smooth. Pour mixture into glass bowl, cover and place in the fridge for at least 1 1/2 hours or overnight.