These cupcakes are adapted from a recipe by ChikaLicious Dessert Bar in New York City. Martha made this recipe on episode 508 of Martha Bakes.
Preheat oven to 350 degrees. Line a standard 12-cup muffin tin with paper liners. In a small bowl, sift together flour, salt, baking soda, baking powder, and matcha.
In an electric mixer fitted with paddle attachment, cream together butter and sugar on medium-high speed until light and fluffy, 3 to 5 minutes. Add egg and mix on medium until combined, scraping down sides of bowl as needed. Reduce speed to low. Add flour mixture and then milk, mixing until just combined.
Divide batter evenly among lined cups, filling each about halfway. Bake, rotating halfway through, until cupcakes spring back to touch and a cake tester inserted in centers comes out clean, about 16 minutes. Let cool in tin 5 minutes, then transfer cupcakes to a wire rack to cool completely.
Fill a pastry bag fitted with a small round tip (Ateco #801) with green tea ganache. Pipe about 1 tablespoon ganache into center of each cupcake. (Reserve leftover ganache to decorate cupcakes.) Fill another pastry bag fitted with a large plain-round tip (Ateco #809) with green tea buttercream. Pipe a dollop of buttercream onto top of each cupcake. Sift a small amount of matcha onto half of each cake. Pipe a squiggle on each cupcake using reserved ganache. Store cupcakes overnight in an airtight container at room temperature or refrigerated for up to 3 days. Bring to room temperature before serving.