What is the easiest way to measure 50ml OF WATER?
The best way to measure 50mL of water is to use a graduated cylinder. These are much more accurate than using beakers and similar containers.
If your recipe is in wieght- use a scale (any kind that has the right units on). If your recipe is in volume (say you want 200ml), use a measuring jug and fill it a little way with water and then add butter until it get 200ml higher than where the water reached. If you have too much butter to measure these ways just do it in parts.