Put the butter in a bowl and knead until smooth. Add the granulated sugar little by little, and mix until it becomes white and pomade-like.
Add the egg yolk, and mix evenly.
Add the sifted flour ingredients and mix with a rubber spatula.
Wrap in plastic wrap, and chill in the fridge for around an hour until hardened.
Sandwich the dough in two sheets of plastic wrap and roll out with a rolling pin to 2mm thickness and then cut out shapes. If the dough becomes soft, chill it in the fridge. If you don't have a cookie cutter, cut into squares with a knife.
Line the cookies on a baking sheet lined with parchment paper, and bake for 13 minutes at 350°F/180°C.
When the cookies have sufficiently cooled on a rack, sandwich with the chocolate melted over hot water. Chill in the fridge, and when they have hardened, they are done!
Story Behind this Recipe
I thought that matcha and white chocolate must surely be perfect together. You can spread chocolate on the cookies too, but if you are giving them away as a present, having them too sticky is not good, so I put the chocolate inside like a sandwich.
When the cookies bake, they turn brown rather than green, so after about 7 minutes, cover with aluminum foil. That way, they will keep their nice green colour. Use the leftover egg white in another recipe, such as Baked Meringue, or Tuiles.