Matcha Whoopie Pies

Directions

1. Preheat oven to 350°F.
2. Whisk together flour, cocoa, baking soda, and salt in a bowl until combined.
3. Stir together buttermilk and vanilla in a small bowl.
4. Beat together butter and brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 3 minutes in a standing mixer, or 5 minutes with a handheld, then add the egg, beating util combined well.
5. Reduce speed to low and alternately mix in flour mixture and buttermilk in batches, beginning and ending with flour, scraping down side of bowl occasionally, and mixing until smooth.
6. Spoon or pipe mounds of batter about 2 inches apart on baking sheets. (I used silpats for these, you can butter the baking sheets or use parchment). Bake in upper and lower thirds of oven, switching position of sheets halfway through baking, until the tops are puffed and the cakes spring back when touched, 11 to 13 minutes if you make them large, 8-10 minutes if smaller. Transfer with a metal spatula to a rack to cool completely.
7. Make filling: Beat together butter, confectioners sugar, marshmallow, Matcha powder, and vanilla in a bowl with electric mixer at medium speed until smooth, about 3-5 minutes.
8. Assemble pie: Spread or pipe filling on flat sides of half of the cakes, and top with remaining cakes. For a large pie, use up to a tablespoon of filling, for the mini pies, use a teaspoonful.