Step 1: Preheat oven to 350°F (175°C). Grease and line an 8-inch springform pan with parchment paper.
Step 2: In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt. Set aside.
Step 3: In a large bowl, cream butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
Step 4: Alternately add dry ingredients and buttermilk to the butter mixture, beginning and ending with dry ingredients. Mix until just combined.
Step 5: Pour batter into prepared pan and bake for 25-30 minutes, or until a toothpick inserted comes out clean. Let cool completely in pan.
Step 6: For the mousse, melt chocolate with 1/2 cup heavy cream in a double boiler or microwave, stirring until smooth. Let cool to room temperature.
Step 7: In a chilled bowl, whip remaining 1 1/2 cups heavy cream with powdered sugar and vanilla until stiff peaks form.
Step 8: Gently fold cooled chocolate mixture into whipped cream until fully incorporated.
Step 9: Spread mousse evenly over cooled cake base. Refrigerate for at least 4 hours, or overnight.
Step 10: Before serving, garnish with chocolate shavings and fresh berries.