Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

Cream together the softened unsalted butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes. Add the egg and lime zest; mix until fully combined.

In a separate bowl, whisk together the flour, baking powder, and kosher salt. Gradually mix this dry mixture into the butter mixture until just combined.

Portion out the dough, roll into balls, and lightly flatten before placing them on the baking sheets, leaving space in between.

Bake for 8-10 minutes until lightly browned on the bottoms and slightly set on the tops. Cool on the sheets for 2 minutes.

Whisk together powdered sugar, lime juice, heavy cream, and optional rum in a medium bowl until smooth.

Dip the tops of each cookie into the glaze, swirl to coat, and return to the wire rack. Optionally sprinkle more lime zest on top.

Let the glazed cookies sit at room temperature to harden before serving.