Step 1: Prepare the Dough

1. Combine Dry Ingredients: In a medium bowl, whisk together the glutinous rice flour, all-purpose flour, Thai tea powder, baking powder, and salt. This helps to evenly distribute the Thai tea powder and baking powder.
2. Cream Butter and Sugar: In a separate large bowl, beat the softened butter and granulated sugar together until light and fluffy. This should take about 2-3 minutes.
3. Add Condensed Milk, Egg, and Vanilla: Beat in the sweetened condensed milk, egg, and vanilla extract until well combined.
4. Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, mixing until a smooth, sticky dough forms. The dough will be quite soft and slightly sticky due to the glutinous rice flour.

Step 2: Chill the Dough

1. Cover and Chill: Cover the bowl with plastic wrap and refrigerate the dough for at least 1 hour. Chilling helps the dough firm up, making it easier to handle and helping the cookies keep their shape while baking.

Step 3: Shape the Cookies

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. Roll into Balls: Scoop out about 1 tablespoon of dough and roll it into a ball between your palms. The dough should be firm enough to handle, but if it’s still sticky, you can lightly dust your hands with cornstarch.
3. Coat with Powdered Sugar: In a small bowl, mix the powdered sugar and cornstarch. Roll each ball of dough in the mixture until fully coated. The powdered sugar will create the beautiful crinkle effect as the cookies bake.

Step 4: Bake the Cookies

1. Arrange on Baking Sheet: Place the coated cookie balls on the prepared baking sheet, spacing them about 2 inches apart. They will spread slightly as they bake.
2. Bake: Bake for 12-15 minutes, or until the cookies have cracked on top and are set around the edges. The centers should still be soft to achieve that chewy mochi texture.
3. Cool: Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Step 5: Serve and Enjoy

1. Serve Fresh: These cookies are best enjoyed fresh, while the exterior is slightly crisp, and the inside is chewy.
2. Store Leftovers: Store leftover cookies in an airtight container at room temperature for up to 3 days.