1. Preheat the oven to 350°. Prepare a standard Bundt pan with baking spray; set aside.
2. In a small saucepan, melt the chopped chocolate over low heat, stirring constantly. When melted, remove from heat and allow to cool for 5 minutes.  Whisk in the oil and sugar until smooth and creamy, then whisk in the egg.
3. In a small bowl, whisk the flour, cocoa powder, baking soda and salt. Set aside.
4. In the bowl of a stand mixer slowly mix in half of the chocolate mixture, followed by half of the milk and buttermilk, then half of the dry ingredients.  Repeat using remaining ingredients.  Do not overmix.
5. Pour the batter into the prepared cake pan and for about 45 minutes, or until a toothpick inserted in the center of the cake comes out with a few crumbs.  Remove from oven and allow to cool in pan for 10 minutes before removing to a wire rack.
6. When cooled completely, cut cake into two halves horizontally.  Fill bottom half with prepared chocolate mousse.  Do not replace top half of cake, instead, wrap it tightly in plastic wrap for later.  Allow bottom of cake and mousse to cool completely in refrigerator before and replacing the top half of cake to make slicing and serving easier.  Top with chocolate gannche, if desired.

Mousse

1. In a medium sized bowl, beat whipping cream to soft peaks.  Add remaining ingredients and beat to stiff peaks.  Chill until needed.