Preheat your oven to 350°F (175°C). Position your rack in the middle of the oven.
Line an 8×8-inch baking pan with parchment paper, leaving some overhang on the sides. Lightly spray with non-stick cooking spray.
Melt the butter and chocolate in a microwave-safe bowl in 30-second intervals, stirring between each until completely smooth.
Add sugar to the chocolate mixture and whisk until well combined. Allow to cool slightly (about 5 minutes).
Whisk in eggs one at a time, then add vanilla extract and mix until smooth and glossy.
Sift together flour, cocoa powder, and salt in a separate bowl.
Fold the dry ingredients into the wet ingredients until just combined.
In a separate bowl, combine the melted butter and sugar for the matcha swirl and whisk until well mixed.
Add the egg and whisk until the mixture is smooth and slightly lighter in color.
Sift the matcha powder into the mixture to avoid clumps.
Add the flour and salt and gently fold until just combined.
Pour about ⅔ of the chocolate brownie batter into the prepared pan, spreading it evenly.
Dollop the matcha mixture on top of the brownie layer, distributing it evenly.
Top with the remaining brownie batter, adding it in dollops between the matcha layer.
Create swirls by running a knife or skewer through the batters in a figure-eight pattern.
Bake for 25-30 minutes or until a toothpick inserted comes out with a few moist crumbs.
Allow the brownies to cool completely in the pan on a wire rack.
Once cooled, lift the brownies out using the parchment paper and place on a cutting board.
Cut into 16 squares.