Cutting butter accurately requires stable footing and cold fat. Soft butter smears across the wrapper, shifting the printed lines and muddying your cut. Keep the butter refrigerated until the moment you slice.
Unwrap the stick partially without removing it completely from the paper casing. Locate the printed tablespoon hash marks. Count down to the line marked 5 tablespoons. Look at the space between the 5-tablespoon mark and the 6-tablespoon mark. Visually divide that single-tablespoon segment into three equal parts.
Position a large, straight-edged chef's knife, not a serrated knife, one-third of the way past the 5-tablespoon mark toward the 6-tablespoon mark. Press straight down in a single, firm motion. If you prefer absolute certainty without guessing the visual fraction, slice cleanly at the 5-tablespoon line, then use a standard measuring spoon to scoop 1 level teaspoon from the remaining block.
