To find the number of well-done burgers Valentina's Burgers served, we start with the total number of burgers served, which is 5. According to the problem, 40% of these burgers were well done.
We can calculate the number of well-done burgers by finding 40% of 5:
\[ \text{Number of well-done burgers} = 0.40 \times 5 \]
Calculating this gives:
\[ 0.40 \times 5 = 2 \]
Thus, the number of well-done burgers served is \( \boxed{2} \).