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

Make the Filling:

• In a small saucepan set over low heat, combine the chocolate chips, condensed milk, and 1 tablespoon of butter. Heat, stirring often, until melted and smooth. Set aside to cool.

Make the Blondies:

• In a large bowl, stir together the melted butter and brown 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.

Assemble:

• Spread half of the blondie batter into the bottom of the prepared pan. Using your hands, pick up portions of the fudge filling, flatten with your palms, and top the batter with pieces of the flattened fudge evenly. Spread the remaining blondie 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 slicing and serving.