Preheat the oven to 350°F (175°C) and line the muffin tin with 12 paper cupcake liners.
In a medium bowl whisk together flour, baking powder, baking soda, salt, and matcha powder for the cupcakes, then set aside.
In a large mixing bowl beat the softened butter and granulated sugar with an electric mixer on medium speed until light and fluffy, about 2–3 minutes.
Add the eggs to the butter mixture one at a time, beating well after each addition, then mix in the vanilla extract.
In a separate small bowl whisk together the milk and sour cream until smooth.
Add one-third of the dry ingredients to the butter mixture and mix on low just until combined.
Add half of the milk mixture and mix on low until just incorporated.
Repeat with another third of the dry ingredients, the remaining milk mixture, and then the final third of the dry ingredients, mixing lightly after each addition.
If desired, add sage green gel food coloring a drop at a time and gently fold with a spatula until a soft sage green color is reached.
Divide the batter evenly among the 12 liners, filling each about two-thirds full.
Bake the cupcakes for 16–18 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Remove the pan from the oven and let the cupcakes cool in the tin for 5 minutes.
Transfer the cupcakes to a cooling rack and let them cool completely for about 25 minutes before frosting.
While the cupcakes cool, prepare the frosting by beating the softened butter in a large bowl until creamy and pale, about 2 minutes.
Gradually add the powdered sugar, 1 cup at a time, beating on low at first, then increasing to medium until combined and fluffy.
Mix in the vanilla extract, matcha powder, salt, and 2 tablespoons of heavy cream, beating until smooth and spreadable.
Adjust the frosting’s consistency with additional heavy cream by the teaspoon if it is too thick, or a little more powdered sugar if it is too thin.
Tint the frosting with sage green gel food coloring a drop at a time, beating after each addition until the desired soft sage hue is reached.
Fit a piping bag with your chosen tip and fill it with the sage green frosting.
Pipe swirls of frosting onto each cooled cupcake, starting from the outer edge and working inward and upward.
Garnish each cupcake with a small fresh sage leaf and a light sprinkle of white sanding sugar, if using.