Chai-Spiced Buttercream Cookie Bars

Preheat oven to 350℉. Line an 8x8-inch baking pan with parchment paper.
In a large bowl, whisk together the melted butter and brown sugar until smooth. Add the egg, brewed chai tea, and vanilla, and whisk to combine.
Stir in the flour, baking powder, cinnamon, ginger, cardamom, and salt until just combined. Do not overmix.
Spread the mixture evenly into the prepared pan. Bake for 18-20 minutes, or until the top is set and a toothpick comes out clean or with a few moist crumbs. Let the bars cool completely.
To make the buttercream, in a medium bowl, beat the softened butter until smooth. Gradually add the powdered sugar, then the chai tea and spices. Beat until light and fluffy.
Spread the buttercream evenly over the cooled bars. Sprinkle with additional cinnamon. Slice into squares and serve.