How to Make It
In a medium bowl, using a hand mixer, beat the butter and sugar on medium speed until light and fluffy, about 2 minutes. Beat in the yogurt until well combined, then beat in the curry leaves, serrano, cilantro and kosher salt. Using a wooden spoon, fold in the flour and cheese until the dough just comes together. Turn the dough out onto a work surface and press into a ball. Roll the dough into a 1 1/2-inch-thick log, wrap tightly in plastic and refrigerate until firm, about 1 hour.
Preheat the oven to 350° and line a large rimmed baking sheet with parchment paper. Slice the dough into 1/4-inch-thick rounds and arrange them 2 inches apart on the prepared baking sheet. Bake for 18 to 20 minutes, until firm and golden around the edges; rotate the pan halfway through baking. Transfer the shortbread to a rack and sprinkle with fleur de del, if using. Serve warm or at room temperature.