Preheat oven to 350ºF. Bring lime juice to a simmer in a small saucepan over medium heat. Cook until reduced by half. Add 1/2 cup butter and stir until melted. Set aside.

In a bowl, whisk together flour, baking powder and salt, set aside. Put remaining 1/2 cup butter and 1 cup sugar in another bowl and with an electric mixer, beat on medium until creamy. Add egg and reserved lime butter; mix until pale about 3 minutes. Mix in vanilla and 2 tsp lime zest. Reduce speed to low and add mix in flour and poppy seeds.

Stir together remaining 1/2 cup sugar and 1-1/2 tsp lime zest in a small bowl. Roll dough into 2-1/2 inch balls. Roll balls in lemon-sugar mixture. On parchment lined baking sheets, place balls about 2 inches apart. Press each with the bottom of a glass dipped in sugar mixture until about 1/4 inch thick. Sprinkle with more poppy seeds.

Bake until just brown, about 12 to 13 minutes. Cool completely on sheets.