These were a fun new recipe we tried. My husband found the recipe in the Sunset magazine. Really good with coffee. Not too sweet. The Matcha Green Tea powder was hard to find so if you decide to make it, make sure you can get it in your area.
Preheat oven to 350°. Sift flour, cocoa, baking soda, and salt into a medium bowl.
Beat butter and brown sugar in a large bowl with a mixer on medium speed until very fluffy, 3 to 4 minutes.
Beat in egg, vanilla extract, and vanilla seeds. Reduce speed to low and gradually add flour mixture; then gradually beat in buttermilk until smooth, scraping down inside of bowl as needed.
Line 2 large baking sheets with parchment paper or Silpat mats*. Put batter in a piping bag with a large tip (or cut a 1/2-in. corner off a resealable plastic bag and spoon in batter). Form 1-tbsp. mounds of batter 2 in. apart on sheets. Bake about 12 minutes or until cakes spring back when poked. Transfer to rack to cool.
Meanwhile, make filling: Sift powdered sugar and matcha together into a medium-large bowl. Add cream cheese, butter, and mascarpone and beat with a mixer on low speed at first, then on medium until very smooth and creamy, about 2 to 3 minutes.
Using an offset spatula or a butter knife, generously spread some filling on the flat side of a cake and top with another cake. Repeat for rest of cakes.
*Chill matcha after opening to slow oxidation. Silpats (silicone baking mats) prevent sticking.
Make ahead: Cakes, up to 3 days, stored airtight at room temperature between sheets of parchment; filling, up to 2 days, chilled (bring to room temperature before spreading).