Preheat the oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.

In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.

Fold in the chocolate chips gently until evenly distributed.

Pour the batter into the prepared cake pan and smooth the top. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean. Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

While the cake cools, prepare the mousse. In a medium bowl, whip the heavy cream and powdered sugar until soft peaks form. Gently fold in the vanilla extract.

Once the cake is completely cool, slice it in half horizontally to create two layers. Place one layer on a serving plate.

Spread half of the whipped cream mousse over the first layer, then top with the second layer of cake. Spread the remaining mousse on top and around the sides of the cake.

Chill the cake in the refrigerator for at least 2 hours before serving to allow the mousse to set.