Melt butter in medium-sized heavy bottom saucepot over medium-low heat. Add 1/4 cup of the milk; stir. Cut VELVEETA into 1/2-inch cubes. Add to saucepot; mix well. Cook over medium heat until VELVEETA is completely melted, stirring frequently. If fondue is too thick for your taste, gradually add remaining 1/4 cup milk until of desired thickness. To serve, spoon fondue over bread slices. Or, cut bread slices into cubes and dip into fondue.