Begin by finely chopping the dark chocolate and placing it in a heatproof bowl.
In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Remove from heat.
Pour the hot cream over the chopped chocolate and let it sit for 2-3 minutes to soften the chocolate.
Add the butter, matcha powder, powdered sugar, and sea salt to the chocolate mixture. Stir gently until everything is completely melted and well combined.
Line an 8-inch square baking dish with parchment paper, leaving some overhang for easy removal.
Pour the chocolate mixture into the prepared dish and spread it evenly.
Refrigerate for at least 2 hours or until firm.
Once set, lift the chocolate out of the dish using the parchment paper and cut it into small squares.
Dust the pieces with additional matcha powder or cocoa powder for presentation, if desired.