Spray pan with BakeEasy spray. Preheat oven to 350°F.
Melt chocolate in double boiler or microwave in a small glass bowl on high power for 1 minute. (NOTE: White chocolate may not appear melted, but stir before microwaving additional time!)
In large bowl, combine cake mix, milk, butter, eggs, vanilla and white chocolate with electric mixer; beat 2-3 minutes. Pour into prepared pan, smoothing with spatula. Bake 45-50 minutes. Cool in pan on wire rack 15 minutes; remove from pan and cool completely.