Preheat the oven to 325º F. Butter the bottom and sides of an 8 inch round cake pan or springform pan. Line the bottom with a parchment paper circle for easy cake removal. 

In a medium pot over low heat, combine the chopped bittersweet chocolate bar, butter, and sugar.  Stir occasionally until everything is melted and smooth. Remove the pot from the heat, stir in the milk and allow the mixture to cool for 10 minutes.

Pour the chocolate mixture into a mixing bowl.

Add the eggs, beating on low with an electric mixer to combine.

Gradually add in the all purpose flour, baking powder, salt and cocoa powder. Mix on low to combine.

Pour the batter into the prepared pan. Bake for 70-80 minutes or until a toothpick inserted in the middle comes out with moist crumbs.

Allow the cake to cool in the pan for 30 minutes, then transfer the cake to a cooling rack to cool completely.

Place the chocolate chips in a microwave safe bowl and microwave for 2 minutes on 50% power. Stir. Continue to microwave on 50% power until melted, roughly another minute. Allow the chocolate to cool for 5 minutes.

Place the butter in a mixing bowl and beat with an electric mixer on high for 2-3 minutes. Add in the melted chocolate, powdered sugar and vanilla extract, beat on medium until combined and fluffy.

Spread the frosting over the cake. Top with fresh fruit or chocolate shavings if desired. Slice and serve.