Preheat oven to 425 degrees.
Make the ganache. Heat cream in a saucepan on low heat until warm (~1 minute). Pour into a bowl with the chocolate and matcha powder. Mix together until well incorporated and put the bowl in the freezer to harden.
Brush ramekins with melted butter and coat the inside with sugar. Place the ramekins on a baking sheet.
Add butter and chocolate to a small bowl and melt in the microwave. Heat in 30 second increments until the chocolate and butter are melted. Set aside.
Mix eggs, brown sugar, and vanilla in a separate bowl.
Pour about 1/3 of the melted chocolate into the egg mixture to temper it. This means we are bringing the temperature of the eggs up, so they don’t cook. Once tempered, add in the rest of the melted chocolate.
Add in 3 tablespoons of flour and mix well.
Add about 1-2 tablespoons of the mixture to your ramekins, so that it just covers the bottom. Then use a 1 teaspoon cookie scoop to drop spoonfuls of the matcha ganache into the center of each ramekin.
Top off the ramekins with the rest of the cake batter until it just reaches the bottom rim.
Bake for 12 minutes.
Let cool for about a minute after it’s out of the oven.
Use a knife to release the cake from the sides of the ramekin. Put a plate on top of the ramekin and flip everything 180 degrees so the ramekin is now standing upside down. The cake should come right out onto the plate.
Sprinkle powdered sugar on top before serving.