In a medium bowl, cream together the butter, sugar and egg until light and fluffy. Stir in the orange juice and orange zest. Sift together the flour, cornstarch and baking soda; stir into the sugar mixture. Mix in the buttermilk and poppy seeds to form a soft dough. Drop dough by tablespoons onto cookie sheets and press a white chocolate chip into the center of each one.
Bake for 15 minutes in the preheated oven, or until just golden. Cool on cookie sheets for about 5 minutes before removing to wire racks to cool completely. Store up to one week in an airtight container.