Preheat oven to 350F and prepare a 12-cupcake pan with cupcake liners.
Combine flour, baking powder, matcha powder, and salt in a medium-sized bowl. Set aside.
In a large mixing bowl, cream butter and sugar until light in color and fluffy, about 2-3 minutes.
Add sour cream and vanilla extract and mix until well combined.
Add half of the dry ingredients to the batter and mix until well combined. Add the milk and mix until well combined. Add remaining dry ingredients and mix until well combined. Be sure to scrape down the sides of the bowl as needed to make sure all ingredients are well incorporated. Set aside.
Add the egg whites to another mixing bowl and whip on high speed until stiff peaks form and you can tip the bowl upside down without spilling. Use either the stand mixer or a handheld mixer for this step.
Gently fold about half of the whipped egg whites into the rest of the cake batter until mostly combined, then add the remaining egg whites and gently fold until well combined and no white streaks are visible.
Fill the cupcake liners about ¾ of the way full using a 3-tablespoon cookie scoop (or a regular spoon). Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs.
Remove cupcakes from the oven and allow to cool for 5 minutes and then move to a cooling rack. Do not add the buttercream until they are completely cool.