Preheat oven to 350 F. Grease a 8.5 x 4.5 loaf pan.
In a medium bowl, mix together the flour, baking powder and salt. This is the dry mixture.
In a stand mixer bowl, beat together butter for about 1-2 minutes.
Add the sugar to the whipped butter. Cream the butter and sugar thoroughly with medium speed for about 5 minutes, until it is light and fluffy.
Add the egg one by one and mix after each addition.
Add 1/3 of the dry mixture, add 1/2 of the milk and mix. Repeat this step. Then finish with the rest of the flour. Scrape all the sides and bottom of the bowl and mix again.
Take out 1 cup of the original batter and put it in a bowl. Mix in the matcha with the spatula. This is the matcha batter.
Pour in the original batter into the loaf pan.
Then add 6-8 dollops of matcha batter on top of the original batter. Use a knife to make the swirl on top of the original batter.
Turn down the temperature to 340 F. Bake for 50 to 60 minutes until an inserted toothpick comes out clean.
Put the loaf pan on a wire rack for 10 minutes then take out the loaf out of the pan. Cool before serving.