Preheat your oven to 350°F (175°C) and grease a 9-inch springform pan, lining the bottom with parchment paper.

In a large bowl, sift together the all-purpose flour, cocoa powder, baking powder, and salt.

In a separate bowl, whisk the eggs and granulated sugar until pale and fluffy, about 3-5 minutes. Gradually add melted butter and vanilla extract.

Fold the dry mixture into the wet ingredients carefully to maintain airiness.

Pour the batter into the prepared pan and bake for 12-15 minutes.

Allow the cake to cool completely in the pan on a wire rack.

Bloom the gelatin in a small bowl with water according to package instructions.

Melt the semi-sweet chocolate and half of the heavy cream over low heat until smooth. Stir in the bloomed gelatin and set aside to cool.

Whip the remaining heavy cream to soft peaks and fold into the chocolate mixture.

Spread the mousse evenly over the cooled cake and refrigerate for at least 2 hours to set.

For ganache, heat remaining heavy cream until simmering, pour over chopped chocolate, and stir until melted.

Pour the ganache over the set mousse and chill again for at least 1 hour before serving.

Release the cake from the springform pan and slice into wedges to serve cold.