Beat butter and sugar until smooth. Add egg yolks, lime zest and vanilla and blend. Stir in flour and salt until evenly combined. Spoon and shape 1-inch balls of dough. In bowl with sugar, roll dough ball until covered. Place 1 1/2 inches apart on prepared baking tray. Press an indentation in the center of each cookie (a few cracks are to be expected) and bake for 13 to 15 min, until cookies just begin to brown lightly on the bottom. Let cool.
Stir apricot jam to loosen and spoon a bit of jam in each thumbprint. Melt white chocolate in microwave on med. heat, stirring every 10 seconds. Dip a fork into chocolate and drizzle over cookies or put melted chocolate in pastry bag and squeeze over cookies. Let cookies set for at least an hour before packing. Refridgerate.
Cookies will keep up to a week in airtight container.