In a medium bowl combine flour, baking powder, salt and key lime zest.
Melt butter, transfer to a large bowl. Add brown sugar and mix until there are no lumps. Allow to cool slightly. Mix in egg, vanilla and lime juice, then add dry ingredients. Stir until just combined, then fold in white chocolate chips.
Line a 9 x 9 inch metal pan with aluminum foil; grease with spray oil. Transfer batter into pan; bake for 15-20 minutes, until edges brown and begin to pull away from foil and a toothpick comes out clean. Allow to cool before cutting into squares.
Drizzle with melted white chocolate if desired.
* To avoid 'sinkage', it is important to use a metal pan. If you must use a glass pan, be sure to line it with foil, and increase the baking time by approximately 5 minutes.