Line 2 baking sheets with parchment paper. Don't use waxed paper.
Mix powdered sugar, matcha powder, and food coloring in a medium bowl. Sift mixture into another bowl and set aside.
Beat egg whites and salt in a glass, metal, or ceramic bowl using an electric mixer until medium peaks form. Sprinkle in white sugar and beat until stiff peaks form. Mix in the matcha sugar until just combined; no color ribbons should remain.
Scoop some batter into a piping bag fitted with a medium round tip. Pipe out 1-inch cookies about 1 inch apart onto the prepared baking sheets. Let sit until dry, 1 to 2 hours.
Preheat the oven to 350 degrees F (175 degrees C).
Bake cookies in the preheated oven until set, 5 to 10 minutes, watching that they do not brown too much.
Place chocolate in a dry glass bowl set over a pot of simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes.
Beat butter in a bowl using an electric mixer until creamy. Add the melted chocolate and mix in powdered sugar. Sandwich a portion of this ganache filling between pairs of cookies.
