Green tea powder, aka matcha and maccha, is the soul of green tea cake. If you want to make good green tea cake, you have to use good quality green tea powder.
GTP has three enemies: heat, light and oxygen. The colour and flavour deteriorate very quickly unless the tea is kept in the cold, in the dark, away from oxygen.
Good matcha is sold in an airtight and lightproof metal container, and it comes with an oxygen absorber. It should be used as soon and as quickly as possible. And it should be refrigerated, even when it's still sealed.
Lousy matcha, OTOH, sits in a see-through plastic container that's sealed but not really airtight. It's essentially hay with artificial colouring and flavouring. It's good if you're baking a cake for your pet rabbit.
To make green tea cake, just take whatever cake recipe you like and substitute a bit of the flour with matcha. Right?
Yes, of course you can do that. In fact, a lot of people do. But matcha and wheat flour are two entirely different things. The former consists of leaves/fibre; the latter consists of protein and starch. Compared to flour, ground up leaves absorb more water but less fat. Unless you adjust the recipe, a cake made with green tea powder would be less fluffy and more oily than one made without.
Besides the absorbancy, there're two more differences between matcha and flour.
Does matcha taste like flour? Of course it doesn't. Green tea powder is slightly bitter but flour isn't. It makes the cake less sweet.
Does matcha brown like flour? Nope, it doesn't. Wet leaves don't brown, so green tea powder makes the cake brown slower unless you add more sugar or increase the oven temperature.
Most cake recipes need a wee bit of salt. You shouldn't be able to taste the salt at all. It's there to round off the sweetness. Green tea cake, however, doesn't need any salt because matcha does the "rounding off". If there's salt, you'd be able to taste it.
The flavour of green tea powder is very delicate. If there's baking soda/powder in the recipe, the delicate flavour would be compromised. Hence, green tea cake should be made without any baking soda/powder.
I don't like green tea cake that's dense, salty, oily, not sweet, not brown and not "matcha-ish". Do you?
My green tea cake is made with a recipe that's dedicated to making green tea cake that's soft, fluffy, and subtly fragrant with the scent of green tea powder. It's not some vanilla cake with the vanilla yanked out and matcha bunged in.
If you want to try my recipe, be warned that the cake cracks even with cardboard insulation to help it rise evenly. The cracks close up nicely as the cake cools down but you'd still see a few lines. Here's what the cake looks like hot from the oven and after cooling down: