Pulse the Oreos in a food processor until finely crushed; transfer to a bowl and stir in the melted butter until evenly combined.
Divide the cookie mixture among 8 small dessert cups or ramekins and gently press into an even layer to form the crust.
Put the cold water in a small bowl and sprinkle the gelatin evenly over the surface; let it bloom for 5–10 minutes.
Meanwhile, pour the heavy cream into a medium bowl and whip with an electric mixer until soft peaks form, then add 1/4 cup (35 g) of the powdered sugar and continue whipping to stiff peaks; set aside.
In a separate bowl, beat the softened cream cheese with an electric mixer until smooth and fluffy (about 2 minutes). Add the remaining 1 1/4 cups (approximately 164 g) powdered sugar and beat until combined.
Add the mint extract, peppermint extract, and food coloring (if using) to the cream cheese mixture and mix until evenly colored and fragrant.
Heat the bloomed gelatin in the microwave on high for 30 seconds, then whisk for 1 minute until fully dissolved; let cool about 3 minutes but do not allow it to set.
Pour the slightly cooled gelatin into the cream cheese mixture and immediately blend with the hand mixer until fully incorporated and smooth.
Fold the whipped cream and the chopped semi‑sweet chocolate into the cream cheese mixture gently until evenly combined.
Transfer the mousse into a piping bag and pipe over the prepared Oreo crusts, or spoon the mousse into the cups to fill them evenly.
Chill the assembled mousses in the refrigerator for 3 hours to set.
Just before serving, optionally pipe sweetened whipped cream on top and garnish with mint leaves and additional chopped chocolate.