Step 1Line a shallow, 8-inch square baking pan with plastic wrap. Melt the chocolates in a large bowl set over a pan of simmering water. Remove from heat and set the bowl aside.
Step 2Sift the matcha powder into a small bowl, making sure there are no lumps. Pour one-fourth cup of the cream into the bowl and stir or whisk to form a smooth paste. 3. In a small saucepan over medium heat, stir the remaining cream and corn syrup. Bring to a gentle boil. Stir in the matcha paste and whisk to completely combine. Remove from heat.
Step 3Pour the hot cream mixture into the bowl of melted chocolate and stir to thoroughly combine. Stir in the butter and mix well. Pour the mixture into the prepared pan; refrigerate overnight.
Step 4The next day, remove the chocolate from the pan and discard the plastic lining. Cut the chocolate into 8 rows vertically and horizontally to make 64 truffles. Serve slightly chilled.