Combine flour, matcha powder, and salt in small bowl. Set aside.
Cream the butter and sugar on medium-high speed until light and fluffy, approximately 3 minutes.
Add egg and vanilla extract to butter mixture.
Add flour mixture to butter mixture, mix until combined.
Remove the dough from the mixer and form it into a smooth ball.
If the dough is crumbly, add one teaspoon of water at a time. If the dough gets too wet, add one teaspoon of flour at a time.
Press the dough into a round disc, wrap in plastic, and refrigerate for at least an hour.
Roll out the dough between two pieces of parchment paper until it is 1/4" thick.
Cut out circles using a large round cookie cutter. Transfer to parchment-lined baking sheet.
Brush each circle with egg wash. Place 1/2 teaspoon of white chocolate filling in center of each.
Fold the left side 1/3 of the way into center. Repeat with the right side. The top of the right fold will overlap the top of the left fold. Repeat with the bottom. Tuck the left side of the bottom fold under the bottom of the left fold.
Pinch all corners. Brush corners with egg wash.
Transfer to refrigerator and let rest for 30 minutes.
Bake Hamantaschen cookies for 10-12 minutes or until they are light golden brown. Let cool completely.
White Chocolate Frosting
Place white chocolate chips and heavy cream in small saucepan and turn heat to low. Cook, stirring continually, until no chocolate chips remain.
Remove from heat and transfer to medium bowl.
Add confectioner's sugar and stir until completely combined.
If you're not a fan of matcha, simply leave it out of the recipe. You may find you'll need to add 1 or 2 teaspoons of water to create a smooth consistency before chilling the dough.