When Lisa squares her favorite $2$-digit number, she gets the same result as when she cubes the sum of the digits of her favorite $2$-digit number. What is Lisa's favorite $2$-digit number?
1. Let \( x \) be Lisa's favorite 2-digit number. We can express \( x \) in terms of its digits as \( x = 10a + b \), where \( a \) and \( b \) are the tens and units digits, respectively.
2. According to the problem, squaring \( x \) gives the same result as cubing the sum of its digits. Therefore, we have the equation:
   \[
   (10a + b)^2 = (a + b)^3
   \]
3. We need to find the values of \( a \) and \( b \) that satisfy this equation. Let's expand both sides of the equation:
   \[
   (10a + b)^2 = 100a^2 + 20ab + b^2
   \]
   \[
   (a + b)^3 = a^3 + 3a^2b + 3ab^2 + b^3
   \]
4. Equating the two expressions, we get:
   \[
   100a^2 + 20ab + b^2 = a^3 + 3a^2b + 3ab^2 + b^3
   \]
5. To solve this equation, we can try different values of \( a \) and \( b \) (since \( a \) and \( b \) are digits, they range from 0 to 9). By inspection, we find that when \( a = 2 \) and \( b = 7 \):
   \[
   x = 10a + b = 10 \cdot 2 + 7 = 27
   \]
6. Let's verify this solution:
   \[
   (10 \cdot 2 + 7)^2 = 27^2 = 729
   \]
   \[
   (2 + 7)^3 = 9^3 = 729
   \]
   Both sides are equal, confirming that \( x = 27 \) is indeed a solution.

The final answer is \( \boxed{27} \).