1. Preheat your oven to 350°F (175°C) and prepare a loaf pan by greasing and flouring it to prevent sticking.
2. Gather all the necessary ingredients and tools to ensure smooth preparation.
3. In a mixing bowl, cream the unsalted butter and sugar together until light and fluffy to incorporate air into the batter.
4. Add eggs one at a time, beating well after each addition to blend the mixture thoroughly.
5. In a separate bowl, sift together all-purpose flour, salt, and optional baking powder to ensure an even mixture and remove any lumps.
6. Divide the batter into two equal parts, leaving one part plain and carefully folding matcha powder into the other.
7. Spoon alternating portions of the plain and matcha batters into the prepared loaf pan to begin creating the marble effect.
8. Gently swirl the two batters with a knife or skewer to create the intricate marble pattern, ensuring not to overmix.
9. Place the loaf pan in the preheated oven and bake for approximately 65-70 minutes. Check for doneness by inserting a toothpick into the center, which should come out clean.
10. If the cake starts browning too quickly on top, tent it with aluminum foil for the final 15 minutes of baking to ensure even cooking.
11. Once baked, allow the cake to cool completely in the pan before removing it to prevent breaking.
12. Serve the cake the next day for optimal flavor, as resting allows the ingredients to meld and develop further.
13. Store leftovers in an airtight container at room temperature for 3-4 days, or refrigerate or freeze to extend shelf life further.