In a mixing bowl stir together crushed Oreos and butter, divide mixture among 8 small dessert cups and gently press into an even layer.
Add water to a small bowl then sprinkle gelatin evenly over top, let rest 5 - 10 minutes.
Meanwhile, pour heavy cream into a medium mixing bowl and whip until soft peaks form. Add 1/4 cup (35g) of the powdered sugar and whip until stiff peaks form (shake excess on beater blades into bowl, no need to rinse), set aside.
Add cream cheese to a separate mixing bowl and mix with electric hand mixer until smooth and fluffy, about 2 minutes. Add remaining 1 1/4 cups (164g) powdered sugar and mix until combined.
Add in mint and peppermint extract, and food coloring if using** and mix until combined, set aside.
Heat gelatin mixture in microwave on high power for 30 seconds then remove and whisk 1 minute to ensure it dissolves well.
Let cool 3 minutes (not longer or it may start to set) then pour gelatin mixture into cream cheese mixture and immediately blend with hand mixer to combined.
Add whipped cream mixture and chopped chocolate to cream cheese mixture and fold just until evenly combined.
Pour mixture in batches into a piping bag and pipe mousse over Oreo crust layer (or just spoon it into cups). Chill 3 hours.
Serve chilled and if desired (just before serving) pipe sweetened whipped cream over top, garnish with mint and chopped chocolate.
*If you don't want to purchase both the mint and peppermint extract you can just use 3/4 tsp of the peppermint extract. I just prefer the blend of the two.
**I used about 16 drops green and 8 drops yellow - just a generic brand but they're probably all about the same.