Line an 8-inch square baking pan with foil; coat with cooking spray. Melt butter in medium saucepan over medium-low heat. Stir in sweetened condensed milk, brown sugar and corn syrup. Cook over medium heat until sugar is melted and mixture comes to a simmer, stirring frequently. Reduce heat to low. Simmer 10 minutes, stirring constantly, until mixture begins to thicken (you will begin to see the stir marks).
Remove from heat; let stand 30 seconds or until bubbles stop. Stir in baking chips; let stand 2 minutes. Stir until melted and smooth. Pour and spread into baking pan; sprinkle with salt. Refrigerate 2 hours or until firm.