Preheat oven to 350°F. Grease a 9-inch square pan and set aside.
Melt chocolate and butter in a saucepan on low heat, stirring constantly. Once melted, remove from heat and let sit for 5 minutes
In a large mixing bowl, beat sugar, eggs and vanilla on medium-high speed for 5 minutes. Pour in chocolate mixture and beat on low until combined. Beat in flour just until mixed. Pour mixture into pan.
Place chocolate bars evenly into the brownie mixture. Gently press down with the back of a spoon to embed.
Bake for 40 minutes. Remove from oven and cool completely before cutting into squares.