In a medium saucepan over low heat, combine the semi-sweet chocolate chips, sweetened condensed milk, and shortening. Stir continuously until the chocolate is melted and the mixture is smooth. Remove it from heat and set aside to cool slightly.
In a large mixing bowl, cream together the butter and packed brown sugar until light and fluffy. This should take about 3-5 minutes. Add the vanilla extract and salt, mixing until well incorporated.
Gradually add the sifted all-purpose flour to the butter mixture. Mix until just combined. If the dough seems too dry, add a splash of milk to help bring it together.
On a lightly floured surface, take half of the dough and roll it out into a rectangle about 1/4 inch thick. Repeat with the second half of the dough.
Spread half of the chocolate filling over one rectangle of rolled-out dough, leaving a small border around the edges. If desired, sprinkle with chopped walnuts or almonds for added texture. Roll the dough tightly, starting from the edge closest to you, to form a log. Repeat with the second piece of dough and remaining filling.
Wrap each log in plastic wrap and refrigerate for at least 2 hours or until firm. This step is crucial for maintaining the shape when slicing.
Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Once chilled, remove the logs from the refrigerator and slice them into 1/2-inch thick rounds. Place the rounds on the prepared baking sheet, leaving space between each one. Bake for 12-15 minutes or until the edges are slightly golden.
Allow the swirls to cool on the baking sheet for a few minutes before transferring them to a cooling rack. Once completely cool, dust with powdered sugar for a beautiful finish.