In a large bowl, mix graham cracker crumbs, cocoa powder, sugar, and melted butter until well combined.
Press the crumb mixture firmly to a 9-inch/22 cm tart or springform pan. Make sure the thickness of the crust is even and the crumbs on the sides of the pan are flush with the rim. Place in the freezer while proceeding.
Thai Tea Cheesecake
Soak the gelatin sheets in cold water until soft. Set aside.
In a medium saucepan, heat Thai tea, 2 cups (500 ml) heavy cream, and sugar until simmering (not boil). Turn off the heat. Squeeze gelatin to remove any excess water and add it to the pan, stirring constantly until the gelatin is melted.
Steep the tea for 10 minutes before straining it through a fine-mesh strainer. Set aside and let the mixture cool completely.
With a hand mixer or stand mixer with a whisk attachment, whip the remaining 1 cup (250 ml) heavy cream until stiff peak. Set aside.
With a hand mixer or stand mixer with a whisk or paddle attachment, mix cream cheese, whipped cream, and vanilla extract until well combined.
Pour the Thai tea mixture into the cream cheese mixture and beat until well combined.
Pour the filling into the prepared crust. Cover with plastic wrap and place in the refrigerator for at least 6 hours or overnight.
Serve with whipped cream on top, if desired. Serve cold.