Bring cream and butter to a boil in a small heavy saucepan and stir in tea leaves. Remove from heat and let steep 5 minutes.
Meanwhile, finely grind chocolate in a food processor and transfer to a bowl. Pour cream through a fine-mesh sieve onto chocolate, pressing on and discarding tea leaves, then whisk until smooth. Chill ganache, covered, until firm, about 2 hours.
Spoon level teaspoons of ganache onto a baking sheet. Put cocoa in a bowl, then dust your palms lightly with it. Roll each piece of ganache into a ball (wash your hands and redust as they become sticky). Drop several balls at a time into bowl of cocoa and turn to coat. Transfer as coated to an airtight container, separating layers with waxed paper.