Green tea cake

Some friends are having a Japanese-themed birthday party and asked me to bake a cake. No further instruction or suggestion beyond "awesome." So, I'm making green tea cake. I practiced today so I don't bring something inedible to the party.

Recipe attempt #1
Loosely adapted from Cook's Illustrated's fluffy yellow layer cake recipe.
10.9 oz. cake flour
3 Tbsp corn starch
1 1/4 tsp baking powder
1/4 tsp baking soda
3/4 tsp salt
4 Tb powdered buttermilk
4 tsp matcha
1 3/4 cups sugar
10 Tb butter, melted and cooled a bit
1 cup water, lukewarm
3 Tb vegetable oil
2 tsp vanilla
6 egg yolks (about 3.75 oz)
3 egg whites (about 3.15 oz)

Preheat oven to 350° F. Prepare two nine-inch round cake pans by greasing the bottoms and sides, cutting a round of parchment paper for the bottoms, greasing the paper, and flouring the inside of the pans.

Mix together the flour, baking powder, baking soda, salt, powdered buttermilk, matcha and 1 1/2 cups of the sugar in a large bowl. Set aside.

Combine butter, water, oil, vanilla, and egg yolks in a medium bowl. Set aside.

Whip the egg whites in a stand mixer fitted with the whisk attachment until foamy. Gradually add the remaining 1/4 cup of sugar, then whip until the whites hold a stiff peak. Transfer the whites to a bowl.

Without bothering to clean the mixer bowl, dump in the flour mixture. Running the whisk on low, add the butter mixture gradually, then mix until just combined. Scrape the sides and mix until all the flour is incorporated (10-15 seconds).

With a rubber spatula, fold in 1/3 of the egg whites to lighten the mixture. Then carefully fold in the rest of the whites until there are no more streaks of white.

Split the batter between the two prepared cake pans. Tap the pans against the counter to release air bubbles.

Bake about 25 minutes, rotating the pans half way through. Cool in the pans on a rack for 10 minutes, then remove the cake from the pans and cool on rack completely.

Frosting

I used Cook's Illustrated's creamy vanilla frosting, plus a teaspoon of matcha.


Problems

I think I over-beat the egg whites a bit, which made them difficult to fold in. But it doesn't seem like this had an impact on the texture of the cake.

I might up the amount of matcha. The batter was a lovely green, but it didn't taste much like green tea. After tasting the actual cake, I think I'll add a teaspoon more.

The side/bottom of the cake is a tiny bit crunchy. I think it will soften after sitting frosted a while, though. I baked the cake for a full 25 minutes. Next time, just 23.

I halved the recipe to just make half a cake (cut it in half and stuck one half on top the other). I also halved the frosting, but it wasn't enough. I'm a bit worried about making this frosting, which CI published alongside a cupcake recipe, since it might not cover. Haven't decided yet what I'm going to do about that.