Melt Chocolate: Melt the dark chocolate and coconut oil together in a double boiler over low heat, stirring occasionally, until smooth and glossy. Alternatively, microwave in a microwave-safe bowl in 20-second intervals, stirring between each interval, until melted and smooth. Set aside.
Prepare Matcha Mixture: Melt the white chocolate and coconut oil together in a small heatproof bowl in the microwave for 20 to 30 seconds, or until melted and smooth. Stir well.
Add Matcha Powder: Add the matcha powder and mix with a spatula until fully combined and smooth. Set aside.
Add Melted Chocolate: Remove the brownie base from the refrigerator and pour the melted dark chocolate over the top. Gently tilt and move the pan in different directions to help the chocolate spread evenly across the surface. If needed, use a spatula to smooth out any areas.
Create Matcha Swirl: Spoon small dollops of the matcha mixture over the dark chocolate layer. Use a wooden skewer or butter knife to gently swirl the matcha through the chocolate, being careful not to overmix.
Chill: Refrigerate for 3-4 hours or overnight, until the chocolate topping is completely set and the brownies are firm enough to slice.
Slice and Serve: Remove the brownies from the refrigerator and let them sit at room temperature for 15 minutes before slicing. For cleaner slices, run a sharp knife under hot water for a few seconds, then wipe completely dry before cutting. Enjoy!