Find the Exact Value of the Expression $\cos 20 ^ { \circ } \cos 40 ^ { \circ } - \sin 20 ^ { \circ } \sin 40 ^ { \circ }$

Multiple Choice

Find the exact value of the expression.
- $\cos 20 ^ { \circ } \cos 40 ^ { \circ } - \sin 20 ^ { \circ } \sin 40 ^ { \circ }$

A) $\frac { \sqrt { 3 } } { 2 }$
B) $\sqrt { 3 }$
C) $\frac { 1 } { 2 }$
D) $\frac { 1 } { 4 }$