Line a 9-inch square pan with parchment paper; butter lightly.
Combine sugar, evaporated milk and corn syrup in a large saucepan over medium-high heat. Stir to combine. Insert candy thermometer; cook, without stirring, until mixture reaches 236-237 degrees F (soft ball stage). Remove pan from heat.
Add cocoa powder, butter, vanilla and salt; stir 60 seconds. Stir in walnuts. Pour into prepared pan.
Let sit 1 hour; cover and let sit at least 4 hours before placing. Slice into 18 3x1-1/2 inch bars.