In a small spice grinder, blitz the tea leaves until finely chopped. Set aside.

In a small saucepan, melt ½ cup of butter over medium-high heat. Cook until foamy and the milk solids turn deep golden brown, about 6-8 minutes. Transfer to a bowl and stir in the vanilla, cardamom, and tea leaves.

Chill the butter mixture in the fridge, stirring every 15 minutes until smooth and spreadable, about 45 minutes. Alternatively, cover it and leave it at room temperature overnight.

In a mixing bowl, combine the cooled butter mixture, remaining ½ cup softened butter, sugar, egg yolk, and salt. Beat on medium speed until creamy, about 1 minute. Gradually add in the flour and cornstarch on low speed until just combined, scraping down the bowl as needed. Shape the dough into a disk, wrap tightly, and chill for 30 minutes.

Roll the chilled dough between two sheets of parchment paper to ¼-inch thickness. Chill again for 1 hour until firm.

Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.

Cut the dough into desired 2-inch shapes and place on the baking sheet 1 inch apart.