Prepare two heatproof containers and soak gelatin powder in cold water for both cheesecake and mousse bases. Bring cream cheese to room temperature and line a bowl with plastic wrap.

For cheesecake base, mix cream cheese until soft, add sugar, and gradually mix in heavy cream. Heat soaked gelatin in microwave for 10-20 seconds and mix into the base along with lemon juice.

Pour cheesecake base into the lined bowl and refrigerate for at least 1 hour until set.

For chocolate mousse base, whisk egg yolks and sugar until pale. Gradually add milk while mixing. Melt chocolate in microwave for 40-60 seconds, mix in gelatin, and cool.

Whip heavy cream until soft peaks form and fold into the chocolate mixture. Pour over the cheesecake layer.

Trim castella cake, slice into triangles, and place on top of the mousse layer. Refrigerate for at least 2 hours until fully set.

For decoration, shave a small amount of chocolate for garnish. Melt remaining chocolate and mix with heavy cream in three additions, whipping until stiff peaks form.