In a small bowl, mix together gelatin powder and water. Let sit for 5 minutes to bloom gelatin.
Place gelatin in microwave safe bowl with milk. Heat milk and gelatin in microwave for 20 second intervals until gelatin has completely dissolved and milk is warm. Set aside.
In a large bowl, melt together dark chocolate and milk chocolate. This can be done in the microwave in 30-45 second intervals at half power. Or, set chocolate bowl over a bain-marie to gently melt. Once chocolate is smooth and melted, let cool for 5 minutes.
Pour warm gelatin mixture into melted chocolate in three intervals. Stir to combine until smooth and homogenous.
In another bowl, whip cold heavy cream to soft peaks. Add whipped cream to chocolate mixture in three addtions. Fold gently. Fold until there are no longer any white streaks of whipped cream left.
Pour chocolate mousse into cooled graham cracker crust. Use a mini offset spatula to spread mousse into an even layer. Place in the fridge to chill for at least 4 hours, overnight is best.