Grease and line a 8-inch (20 cm) springform pan (with a removable base) with parchment paper. Separate egg yolks from whites. Preheat oven to 180°C (350°F).
Melt chocolate and butter over a bain-marie until smooth. Let it cool slightly. Add yolks one at a time and stir well.Stir in cocoa powder and salt.
Whip egg whites until foamy using a mixer. Gradually add sugar and whip until stiff peaks form. Gently fold the whipped whites into chocolate mixture.
Pour batter into prepared pan and bake for 25 minutes. Let it cool completely in the pan over a cooling rack.
Prepare the coffee mousse. Stir egg yolks with sugar until creamy and pale yellow colored. Stir in cornstarch. Gradually whisk in the warm milk until completely combined.
Place the yolks mixture into a small saucepan and cook over low heat while stirring until boils and thickens.
Remove from heat and stir in vanilla extract and instant coffee. Let it cool.
Dissolve gelatin in 2 tbsp cold water and let it swell for about 5 to 10 minutes. Place over low heat just until the gelatin dissolves and then pour it over coffee mixture.
Whip the the cold whipping cream until stiff peaks form. Gradually incorporate into coffee mixture.
Spread the coffee mousse over the chocolate cake in the pan, using an offset spatula.
Cover and refrigerate for about 4 hrs or overnight to set before serving.
Run a warm knife around the edges of the cake to remove the sides of the pan.
Dust the top of the cake with cocoa powder.
Using a warm knife cut the cake into slices and serve.