Preheat the oven to 350°F. Line an 8 by 8-inch pan with foil or parchment. Spray with nonstick cooking spray.

In a small saucepan set over low heat combine the condensed milk, chocolate chips, and butter. Heat, stirring often, until melted and smooth. Let cool.

In a large bowl stir together the butter and sugar. Stir in the egg and vanilla until smooth. Add in the salt and flour and stir until just combined. The mixture will be thick.

Spread half of the batter into the bottom of the prepared pan. Scoop and flatten the fudge mixture evenly over the dough. It doesn’t have to be perfect. Spread the remaining batter over the fudge evenly to cover completely.

Bake for 25 minutes, or until lightly golden on the outside but still fudgy and moist on the inside. It’s better to under-bake than over-bake here! Let cool completely before cutting.