Preheat oven to 350°F (175°C). Grease two 8-inch round pans.
Whisk flour, baking soda, salt, cocoa powder, and 2 cups sugar in a bowl.
In another bowl, mix buttermilk, oil, eggs, and vanilla until smooth.
Gradually combine wet and dry ingredients, mixing until just blended.
Divide batter between prepared pans. Bake 30-35 minutes or until centers spring back. Cool completely.
In a saucepan, melt chocolate over low heat, stirring until smooth. Set aside to cool slightly.
Whip 1 ½ cups cream until softly peaked. In a separate bowl, whip egg yolks, ½ cup sugar, and ¼ cup cocoa powder until light and fluffy.
Gently fold whipped cream into cooled chocolate mixture.
Carefully split each cake layer in half horizontally. Place one layer in a springform pan, spread half the mousse, cover with another layer.
Repeat with remaining mousse and top with final cake layer. Chill 4 hours.
For ganache: Heat ½ cup cream until steamy. Withdraw from heat, add chopped chocolate, and stir until smooth. Let cool slightly.
Pour ganache over chilled cake. Chill 2 hours before serving.