Preheat the oven to 350 degrees Fahrenheit.

Place baking chocolate and butter in a microwave-safe bowl.

Microwave on high for 2 minutes, stirring every 30 seconds until smooth.

Stir in sugar until well combined.

Add eggs one at a time, mixing well after each addition.

Stir in vanilla extract.

Gradually add flour and mix until just combined.

Grease and flour a 9x13-inch baking pan.

Pour the batter into the prepared pan and spread evenly.

Press Hershey's Symphony candy bar squares into the batter.

Bake for 30 to 35 minutes, or until the edges pull away from the sides of the pan.

Remove from the oven and let cool completely in the pan.

Cut into squares and serve.