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.