Place a rack in the center of the oven and preheat the oven to 350. Lightly mist three 8 inch loaf pans with vegetable oil spray, then dust with flour. Shake out excess flour.
Heat the white chocolate in saucepan over low heat until melted, stirring constantly. Set aside to cool slightly.
Place cake mix, melted butter, milk, eggs, vanilla and slightly cooled white chocolate in mixing bowl. Blend with mixer on low speed for 1 minute. Stop machine and scrape down sides of the bowl. Increase the mixer speed to medium and beat 2 minutes more, scraping down sides again, if needed. The batter should look well blended. Divide batter evenly among the prepared pans. Place pans in oven.
Bake the loaves until they are light brown and spring back when lightly pressed with yout finger, 40-45 minutes. Remove pans from oven and cool on wire racks for 10 minutes. Run a dinner knife around the edge of the loaves and invert them onto racks. Turn on one side and let cool completely, 35-40 minutes, before serving.