I love green tea desserts, because the green tea powder adds both a green colour and a gentle flavour. Some recipes call for “green tea for baking”. Ignore that – that type of tea is usually a faded green and will have a weak flavour. While you don't need to buy the very expensive green tea used for tea ceremonies, use the best powdered version that you can afford.
The black sesame paste used for this recipe shouldn’t be too sweet or oily; it should be thick but spoonable, and without a layer of oil floating on top.
Preheat the oven to 180°C (350°F). Prepare one or two loaf tins (with a total capacity of about 3 litres [3 quarts]) by spraying them with pan coating, then lining with baking paper, leaving some overhang.
Use a small sieve to sift the green tea powder into a small bowl.
Put the butter, sugar, salt and baking powder into a large bowl and use an electric mixer on medium-high speed to beat the ingredients until fluffy and pale yellow.
With the mixer speed on low-medium, add the eggs one at a time, letting each incorporate fully before adding the next. Scrape the mixing bowl and beaters with a rubber scraper then mix briefly on low speed.
Add half of the flour and mix on low until amost fully incorporated, then add the yogurt and stir briefly. Scrape the bowl and beaters, then add the remaining flour. Mix on low until almost incorporated, then remove the beaters from the mixer. Scrape the beaters, then use the rubber spatula to finish mixing the cake batter by hand; do not overmix.
Put about one-third of the cake mixture into a medium-size bowl. Out of that amount, remove about one-quarter and place it in a smaller bowl, then mix in the black sesame paste. When it is evenly incorporated, add it back to the larger amount and mix gently until combined, but do not overmix.
Out of the main bowl, remove about one quarter of the batter and place in a smaller bowl. Add the sifted green tea powder and mix until smooth. Add this back into the main cake batter and stir until just combined.
Spoon one-half of the green tea batter into the prepared pan(s). Dollop the black sesame mixture over the green tea cake batter in the pan(s). Top with the remaining green tea mixture. Use your fingertips to marble the two mixtures together in the pan, then smooth out the surface.
Bake at 180°C/350°F for about 45 minutes, or until the cake is fragrant, starts pulling away from the sides of the pan and is firm to the touch.
Cool the cake(s) for about 30 minutes, then take them out of the pan(s) by lifting using the overhang of baking paper. Cool completely before slicing.