“Noce di Burro” is an Italian imprecise measurement for butter.
A “noce” in Italian is a walnut; “burro” means “butter.”
Consequently, the expression means a piece of butter the size of a walnut (unshelled.)
The English equivalent might be “butter the size of a nut”, a measurement which Mrs Beeton used, though we’re not yet certain what kind of nut she pictured.
Equivalents
Roughly 2 tablespoons.