Heat the cream with the butter in a small saucepan over medium heat until it comes to a boil. Remove the pan from the heat and stir in the gunpowder tea and mint leaves. Leave to steep for 30 minutes.
Strain the cream and butter mixture thoroughly, discarding the tea and mint leaves, return to the saucepan and set over medium-low heat. Add the chocolate and vanilla and heat for about 4 minutes, stirring, until the chocolate has melted and the mixture is smooth. Remove the pan from the heat and leave the mixture to cool for a few minutes.
Whisk the sugar, egg yolks and salt together in a large bowl using a hand-held electric mixer until pale in color. Pour in the chocolate mixture while stirring constantly and continue stirring until smooth and well combined.
Divide the mixture between 4 small pots or ramekins and chill in the fridge for about 1 hour until firm. If you keep the pots in the fridge for more than 2 hours, leave them at room temperature for 30 minutes before serving.