Prepare an 8x8” square pan with parchment paper. For thicker slices use a loaf pan.
Warm the dark chocolate in the microwave for about 1 minute. Stir and leave for a minute then return to the microwave for another minute. Stir until smooth.
Pour the sweetened condensed milk and chips into a pot and stir. Turn the heat on low and cook, stirring frequently, until the chocolate is all melted and the mix is smooth. Turn the heat off and add the extract, paste, salt and food coloring. Stir until smooth.
Pour mixture into the prepared pan. Dollop large circles of dark chocolate over the fudge. Use an offset spatula or a toothpick to swirl it. Cover with foil and set in the fridge to chill, about 4 hours.
Slice with a chef's knife. Store in an airtight container. In the fridge it will last about 2 - 3 weeks, at room temp about 1 week.