Line 12 muffin cavities with paper cupcake liners.
Divide the second dark chocolate mixture between the muffin cavities so it fully covers the matcha mixture. Top with flaky sea salt, if using. Set back in the freezer until fully set, at least an hour.
*If the mixture clumps, do not worry, it will still turn out and taste great. Just divide the thicker mixture between each muffin cavity and press it down evenly on top of the first chocolate layer. Set in freezer for at least 5 minutes before adding final chocolate layer.