Preheat the oven to 350 degrees F.
Grease a 8"×8" baking pan with cooking oil, then line with parchment paper. Set aside.
In a small saucepan, heat the butter over high heat until it melts. Remove from heat.
Add white chocolate and matcha powder to a small bowl. Pour the hot butter over the top and stir until all the chocolate melts and the matcha is dissolved. Let the mixture cool for 2 minutes.
In a large mixing bowl, combine granulated sugar, brown sugar, eggs, vanilla extract, and salt. Using a hand mixer or stand mixer, mix until thick and creamy (5-10 minutes).
Add the matcha chocolate mixture into the bowl. Mix until a uniform green batter appears. The color should turn a creamy green (5-10 minutes).
Sift flour into the bowl. Using a spatula, gently fold the flour into the batter until all ingredients are mixed. Optional to fold in white chocolate chips or black chocolate chips or black sesame.
Spoon into your prepared pan.
Bake for 20 minutes.
Take out the brownies and slam them on the counter to reduce some air bubbles. Sprinkle some flaky sea salt over the top and put the brownies back in the oven. Bake for another 20 minutes.
Let cool for 20 minutes before serving. Cut using a sharp knife for a cleaner slice.