Combine flour, sugar, baking powder, and salt in large bowl.
Microwave chips and butter in medium uncovered bowl on High for 1 minute, stir. If necessary, microwave at additional 10-15 sec intervals, stirring until just melted. Cool to room temperature. Stir in milk, eggs, and vanilla extract. Add chocolate mixture to flour mixture. Stir. Batter will be thick.
Cook in waffle maker according to manufacturer's instructions.