A basic buttercream gets an earthy infusion with the addition of fragrant matcha powder. Martha made this recipe on episode 508 of Martha Bakes.
Prepare an ice bath. Combine sugar and 1/2 cup water in a small saucepan. Cook, stirring occasionally, over medium-high heat until sugar is dissolved, 3 to 5 minutes. Transfer syrup to ice bath, stirring occasionally, until syrup is cool to touch, 15 to 20 minutes. (Syrup can be stored in the refrigerator for up to a week.)
In an electric mixer, cream butter on medium-high until light and fluffy, 3 to 5 minutes. While mixing, slowly add syrup into butter and continue mixing on medium-high until mixture comes together, scraping down sides of bowl as needed. Reduce speed to low and add matcha. Store buttercream at room temperature if using the same day, or refrigerate in an airtight container for up to 3 days. Bring to room temperature before using.