Over a double broiler add the unsalted butter and the chopped bittersweet chocolate, then the chocolate chips. Continue to stir the chocolate on low heat until completely melted and smooth. Once melted take off the stove and cool a bit.
In a medium bowl sift the coco powder then add the sugar and whisk. Add the eggs and stir until smooth.
Add the chocolate to the batter a little at a time, you don't want to scramble the eggs in the batter. Add a few tablespoons first, mix the batter then add about half a cup of batter. Continue to add about a half a cup of batter until it is gone. Stir until incorporated, then add vanilla and stir.
For the tart pan cut a piece of wax paper in the shape of the bottom of the tart pan. Grease the wax paper with butter. Pour the batter into the pan and bake for about 20 to 30 minutes.
Let the cake cool for about 30 minutes, then invert the cake onto a dish to remove the wax paper. Invert the cake again so the cake tart design edges are on top. Dust with confectioners' sugar and serve with raspberries on the side.