Preheat oven to 350°. Line several rimmed baking sheets with parchment paper.
In a large bowl, beat together butter and sugar with a mixer at medium speed until light and creamy, approximately 5 minutes. Add egg and vanilla extract, beating until incorporated.
In a medium bowl, whisk together flour, tea leaves, baking powder, and salt. With mixer at low speed, gradually add flour mixture to butter mixture, beating until dough comes together.
Place half of dough between 2 sheets of wax paper. Using a rolling pin, roll out dough to a ¼-inch thickness. Transfer dough and wax paper to another rimmed baking sheet. Freeze for 15 minutes. Repeat with remaining half of dough.
Working with one portion of dough at a time, remove dough from freezer, and remove wax paper.
Using a 2-inch fluted round cutter, cut 30 rounds from each chilled dough portion. Place rounds 2 inches apart on prepared baking sheets.
Bake until edges of cookies are very light golden brown, approximately 10 minutes.
Transfer cookies to wire cooling racks, and let cool completely. Store in an airtight container with sheets of wax paper between layers.
Notes
*If using tea bags, cut tea bag open, and use tea as is. If using loose tea, grind tea in a mortar and pestle until coarsely ground.
Recipe by TeaTime Magazine at https://teatimemagazine.com/jasmine-green-tea-shortbread-cookies/