Using softened butter is very important so it can whip and combine with the powdered sugar. To soften, let set out at room temperature for about 1 hour. Resist the temptation to microwave, which will melt the butter and make it too runny.
Add vanilla and softened butter to a large mixing bowl and beat with a mixer on high for 30 seconds. Then add in powdered sugar 1/2 cup (56 g) at a time being sure to sift as you add, or it will clump.
Add matcha 1 tsp at a time and beat again to combine. The more matcha you add, the more intense the green color and the matcha flavor will become. We found 4 tsp (or more) to be about right (as the recipe is written). Add more as needed for more color and flavor.
If you’re using your buttercream for cakes, it should work well as is. But if using for sugar cookies, you may want to thin it a little bit with dairy-free milk of choice. Add 1 Tbsp (15 ml) at a time and beat until the desired texture is reached.
Use immediately or store in the refrigerator, where it will thicken / harden. Leftovers will keep in the refrigerator up to 1-2 weeks (sometimes longer), or in the freezer up to 1 month (let thaw at room temperature until soft).