Preheat oven to 350°F (175°C). Line a 9x13 inch baking dish with parchment paper.

In a medium bowl, cream together softened butter and granulated sugar until fluffy. Mix in the flour and salt until combined.

Press the dough evenly into the prepared baking dish. Bake for 15-20 minutes or until the edges are golden.

In a saucepan over medium heat, melt the additional butter and stir in the sweetened condensed milk. Bring to a simmer, stirring constantly, until thickened, about 5-7 minutes.

Pour the caramel mixture over the baked shortbread crust. Let it cool at room temperature.

Melt chocolate chips in a microwave-safe bowl or over a double boiler. Spread the melted chocolate evenly over the cooled caramel layer.

Allow chocolate to set completely before slicing into bars and serving.