1-Preheat oven to 350°F (177°C). Grease an 8.5 x 4.5 x 2.75 inch loaf pan.

2-In a mixing bowl, cream together butter, sugar, salt, and baking powder until light and fluffy, scraping the bowl occasionally.

3-Add eggs and egg yolk one at a time, scraping down the bowl after each addition.

4-In a separate bowl, mix sour cream and vanilla extract. Add half of this mixture to the batter and mix.

5-Add half the flour and mix; repeat with the remaining sour cream mixture and flour until just combined.

6-Fold with a spatula 5 6 times to deflate bubbles without overmixing.

7-Split batter in half. Sift matcha powder into one half, add water, and mix until uniform green.

8-Spoon half the vanilla batter into the pan, leaving spaces. Add half the matcha batter into the spaces and swirl gently. Repeat with remaining batter.

9-Tap the pan to remove air bubbles and even the batter. Wet a knife and score the batter down the middle.

10-Bake for 60 70 minutes until a toothpick inserted comes out clean and the cake is golden brown.

11-Let the cake rest in the pan for 10 minutes before removing.