In a double broiler over low heat or in the microwave, melt chocolate chips and butter, stirring occasionally. Whisk in the cocoa.
In a bowl, beat together eggs and sugar at medium-high speed.
Reduce mixer speed to low and add vanilla and salt until combined.
Keep mixer on low and slowly pour the melted chocolate mixture into the egg mixture.
Stir in the flour with a wooden spoon until just combined.
Add chopped peppermint bark and stir until it's mixed in evenly.
Pour the batter into the prepared pan, spreading it into the corners and leveling the top with a rubber spatula.
Bake 30-35 minutes, or until the top is shiny and a toothpick inserted into the center comes out with a small amount of stick crumbs clinging to it.
Set aside to cool.