。
She buttered the bread.
Literal
She [topic-は] bread [on-に] butter [object-を] applied.
塗る ('to spread, apply, paint') covers any thin-layer application — paint on a wall, sunscreen on skin, butter or jam on bread. に marks the surface receiving the layer: パンに塗る ('apply to bread'), 顔に塗る ('apply to the face'). Where English compresses 'butter' into a verb ('to butter the bread'), Japanese keeps the structure verb-explicit: 'apply butter to the bread.'