For the Chai Spiced Sugar Cookies:
In medium bowl, sift together the flour, salt, baking powder and spices. Set aside.
Add butter and sugar to bowl of electric mixer. Beat on medium speed until light and fluffy, about 3 minutes. Add egg, yolk and vanilla bean paste. Beat for 1 additional minute. Reduce speed to low and add dry ingredients. Mix until just incorporated, being careful to not over mix.
Divide dough in half. Shape into 2 logs and wrap tightly in plastic wrap. Chill for at least 2 hours.
Preheat oven to 350°F. Line two baking sheets with parchment paper. Set aside.
Use sharp knife to slice dough logs into 1/4 inch slices. Place on baking sheet, leaving 1 1/2 inches in between. Bake 9-11 minutes. Cookies should be set, but not browned. Let cool on baking sheet for 2 minutes, then transfer to wire rack to cool completely.
For the Chai Tea Buttercream:
Add milk to microwave-safe container and heat until just boiling, about 45 seconds. Add tea, cinnamon stick and cardamom pods. Let cool completely. Strain out tea and spices.
Add butter to bowl of electric mixer. Beat on medium high speed until butter is pale and fluffy. Add half of sugar and beat until incorporated, about one minute. Add remaining sugar, vanilla bean paste salt and infused milk. Beat for another 3 minutes.
To make sandwiches, spread a generous amount of buttercream between two cookies. Roll sides in nonpareils, if desired, and store leftovers in an airtight container in refrigerator.