In a heatproof bowl set over a pan of gently simmering water, melt the white chocolate, stirring frequently until smooth. Set aside.
In a bowl, cream together the butter and sugar until smooth. Beat in the egg and vanilla.
Combine the flour, baking powder and salt; stir into the creamed mixture. Divide dough in half. Mix orange zest into one half, and melted white chocolate into the other half. Use a bit of each mixture to form a single ball about 2.5cm in diameter. (I get a cookie cutter and put the two halves in it so they all turn out the same.) Place on baking tray lined with parchment.