Increase oven temperature to 375℉/190℃.
In a saucepan over low heat, melt butter and semisweet chocolate.
Stir to combine.
Set aside to cool.
In the bowl of an electric mixer, combine eggs, sugar, and vanilla.
Stir to combine.
Beat on high speed 10 minutes.
While egg mixture is beating, whisk together flour and cocoa powder. Set aside.
Reduce mixer speed to low and add melted chocolate mixture.
When combined, stir in flour mixture.
Add bittersweet chocolate chips.
Pour over graham cracker crust.
Bake for 40 minutes, or until done.