Preheat your oven to 170°C. Line a 5-inch square baking pan with parchment paper, making sure to cover the bottom and sides for easy removal later.

Place 120 g dark chocolate and 70 g unsalted butter in a heatproof bowl. Set the bowl over a pot of gently simmering water. Stir until the chocolate and butter are completely melted and the mixture is smooth and glossy. Remove from heat and let it cool until just warm to the touch.

In a large bowl, combine 3 room-temperature eggs and 50 g granulated sugar. Whisk with a hand whisk until the sugar dissolves and the mixture becomes pale and uniform. Pour the cooled chocolate-butter mixture into the egg mixture and whisk until fully combined and smooth.

In a separate small bowl, whisk together 70 g cake flour, 10 g cocoa powder, and 3 g baking powder. Sift the dry ingredients directly into the chocolate-egg mixture. Use a silicone spatula to fold gently until no dry flour remains and the batter is smooth. Be careful not to overmix—overmixing can develop gluten and result in a tough brownie.

Pour the batter into the prepared pan and use a spatula to spread it evenly. Place the pan in the middle-lower rack of the oven and bake at 170°C for 25 minutes. The brownie is ready when the surface is slightly cracked and it springs back when lightly pressed. Remove from the oven and let the brownie cool completely to room temperature.

Place 80 g room-temperature cream cheese in a mixing bowl. Add 10 g granulated sugar and beat with an electric hand mixer until the mixture is smooth, creamy, and completely free of lumps.

Pour 70 g heavy cream into the cream cheese mixture and continue beating until it reaches a thick, yogurt-like consistency. Set aside.

In a small bowl or measuring cup, combine 40 g hot milk with 4 g matcha powder. Whisk vigorously until the matcha is fully dissolved and no lumps remain.

While the matcha milk is still hot, soak 4 g gelatin sheets in a bowl of cold water for 5–10 minutes until softened. Remove the gelatin sheets from the cold water and squeeze out any excess liquid. Add the softened gelatin to the hot matcha milk and stir until completely dissolved. Allow this mixture to cool until just warm to the touch.

Slowly pour the cooled matcha-gelatin mixture into the cream cheese mixture. Use a spatula to fold gently until the mousse is smooth, uniform, and free of streaks.

Pour the matcha mousse over the completely cooled brownie layer. Use a spatula to spread it evenly to the edges. Place the pan in the refrigerator and chill for at least 4 hours, or until the mousse is fully set and firm to the touch.

Once the brownie is fully chilled and set, remove it from the pan by lifting the parchment paper. Carefully peel away the paper and place the brownie on a serving plate. Dust the top of the mousse layer evenly with matcha powder.

To make the chocolate ganache, place 30 g dark chocolate in a small bowl. In a small saucepan, heat 30 g heavy cream over low heat until small bubbles appear around the edges. Pour the hot cream over the chocolate and let it sit for 1 minute. Stir gently until the mixture is smooth and glossy. Allow the ganache to cool until it thickens to a pipeable consistency, then transfer it to a piping bag.

Pipe the ganache onto the surface of the brownie in your desired pattern.

Add chocolate decorations if using. Your matcha brownie is now ready to serve.