Preheat oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt.

In a separate bowl, whisk together eggs, milk, oil, and vanilla extract.

Add the wet ingredients to the dry ingredients and mix until just combined. Stir in the boiling water; the batter will be thin.

Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cake cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely.

Melt the chopped semi-sweet chocolate in a heatproof bowl over a saucepan of simmering water (double boiler method), or in the microwave in 30-second intervals, stirring until smooth. Let it cool slightly.

In a large bowl, whip the cold heavy cream and powdered sugar with an electric mixer until stiff peaks form. Stir in the vanilla extract.

Gently fold about one-third of the whipped cream into the cooled melted chocolate to lighten it. Then, carefully fold the chocolate mixture into the remaining whipped cream until just combined and no streaks remain.

Once the cake has cooled completely, place it on a serving plate. Pour the chocolate mousse evenly over the top of the cake, spreading it to the edges.

Chill the cake in the refrigerator for at least 2 hours, or until the mousse is set.

Garnish with chocolate shavings before serving, if desired.