• Preheat the oven to 160 C / 325 F / Gas Mark 3. Line baking trays with parchment paper. Set aside.
• Place the butter in a large bowl and beat until smooth with a hand whisk. Then add the sugar and whisk until well blended.
• Add 1/3 of the egg white at a time and whisk until incorporated after each addition.
• Sift the flour and the matcha green tea powder into the mixture and mix until well blended with a rubber spatula.
• Transfer the mixture into a piping bag fitted with a 1 cm (1/2 inch) round nozzle. Plastic storage bag also can be used with a corner cut off.
• Pipe 3 cm (1 inch) circle of the dough on the parchment paper, leaving enough space around the dough.
• Bake for 8 minutes until slightly browned in the edges. Then leave them in the oven door closed for 5 minutes. Add a few extra minutes to the bake time, if it's not light golden-brown in the edges.
• Remove the trays from the oven and let them cool. While the cookies are cooling, prepare the white chocolate filling (follow the instructions below). 

White chocolate filling

• Pour 6 cm (2-3 inches) of water in a large pan over high heat to make a double boiler.
• Chop the chocolate into small pieces with a knife, then place in a completely cleaned heatproof bowl.
• Remove the pan from the heat right before boiling point.
• Place the bowl over the hot-water bath and gently stir with a heat proof spatula.
• Remove the bowl from the hot-water bath when the chocolate is about three-quarters melted.
• Stir until smooth, then spoon the chocolate in between two cooled cookies.