Bake the chocolate cake according to the package or recipe directions and let it cool completely. Cut into cubes or crumble.
Whisk together the instant chocolate pudding and cold milk; let set in the fridge for 5–10 minutes.
Melt chocolate chips and allow to cool slightly. Whip heavy cream to stiff peaks and fold in the melted chocolate.
Beat cold whipping cream with powdered sugar and vanilla until fluffy. Add dissolved gelatin for stability if desired.
In a trifle bowl, layer cake, pudding, mousse, and whipped cream. Sprinkle with chocolate chips or crushed cookies and repeat until reaching the top. Finish with whipped cream and garnishes.
Notes
Chill the assembled dessert for at least 4 hours before serving for the best flavor and texture.