In a spacious bowl, mix together all-purpose flour, matcha powder, and coarse kosher salt.

In a separate bowl, cream the unsalted butter and sugar until fluffy, then add the large egg and vanilla extract, blending until smooth.

Gradually mix the dry ingredients into the wet mixture until a smooth ball of dough forms. Wrap the dough in plastic wrap and refrigerate for at least 1 hour.

Once chilled, roll out the dough on a parchment-lined surface to about 1/4" thickness.

Use a cookie cutter or a glass to cut out circles from the rolled dough and place them on a parchment-lined tray.

In the center of each circle, place about 1/2 teaspoon of white chocolate filling.

Carefully fold the sides of the dough towards the center and pinch the corners together to secure the filling inside.

Chill the shaped cookies in the refrigerator for 30 minutes. Preheat your oven to 350°F, then bake for 10-12 minutes until golden brown.