It’s easy to measure butter for recipes calling for warm butter: let it soften, then scoop with a measuring spoon and level off. But if a recipe calls for a quantity of cold butter, and you don’t have a kitchen scale, you can try the water-displacement method.
Fill a liquid measuring cup with water equal to the amount of butter you need (e.g., if you need 1/2 cup butter, put 1/2 cup water in the measuring cup), and add chunks of butter until the water doubles its original volume (in this example, to 1 cup). Just be sure your measuring cup is large enough to accommodate twice the volume that must be measured out.
The water-displacement method also works well for measuring solid shortening and peanut butter. It’s scientifically sound (it uses the Archidemes principle!), and accurate for baking.
Board Links: How to Measure Butter from a Block of it