For what pairs of natural numbers $(a, b)$  is the expression $$(a^6 + 21a^4b^2 + 35a^2b^4 + 7b^6) (b^6 + 21b^4a^2 + 35b^2a^4 + 7a^6)$$  the power of a prime number?
1. **Initial Setup and Assumptions:**
   We start with the expression:
   \[
   (a^6 + 21a^4b^2 + 35a^2b^4 + 7b^6)(b^6 + 21b^4a^2 + 35b^2a^4 + 7a^6)
   \]
   We need to find pairs of natural numbers \((a, b)\) such that this expression is a power of a prime number.

2. **Simplification Using GCD:**
   Let \(d = \gcd(a, b)\). Then \(d = p^i\) for some prime \(p\) and non-negative integer \(i\). We can assume \((a, b) = 1\) (i.e., \(a\) and \(b\) are coprime) without loss of generality. Let \(u = a^2\) and \(v = b^2\). Thus, \((u, v) = 1\).

3. **Expression Transformation:**
   We need to analyze:
   \[
   p^k = f(u, v)f(v, u)
   \]
   where:
   \[
   f(u, v) = u^3 + 21u^2v + 35uv^2 + 7v^3
   \]

4. **Analyzing \(f(u, v) + f(v, u)\):**
   We verify:
   \[
   p \mid f(u, v) + f(v, u) = 8(u + v)(u^2 + 6uv + v^2)
   \]
   If \(p \mid u + v\), then:
   \[
   f(u, v) \equiv 8u^3 \pmod{p}
   \]
   Since \(p \nmid u\) (otherwise \(p \mid u + v\) implies \(p \mid v\), contradicting \((u, v) = 1\)), we have \(p \mid 8\). Thus, \(p = 2\).

5. **Case Analysis:**
   - If \(p \mid u^2 + 6uv + v^2\), then:
     \[
     p \mid 7u^3 + 42u^2v + 7uv^2 \implies p \mid 7u^2v - 14uv^2 - v^3 = v(7u^2 - 14uv - v^2)
     \]
     Since \(p \nmid v\) (otherwise \(p \mid u\)), we get:
     \[
     p \mid 7u^2 - 14uv - v^2
     \]
     Given \(p \mid u^2 + 6uv + v^2\), we have:
     \[
     p \mid 8u^2 - 8uv = 8u(u - v)
     \]
     If \(p \nmid 8\), then \(p \mid u\) (impossible) or \(p \mid u - v\), leading to:
     \[
     u^2 + 6uv + v^2 \equiv 8v^2 \pmod{p}
     \]
     Thus, \(p = 2\).

6. **Special Case \(u = v\):**
   If \(u = v\), then \(u = v = 1\) is the only solution, which indeed works.

7. **General Case \(u \neq v\):**
   Assume \(u > v\) without loss of generality. Since \(f(v, u) - f(u, v) > 0\), we get \(f(u, v) \mid f(v, u)\). Given \(f(v, u) < 7f(u, v)\) and both are powers of two, we have:
   \[
   f(v, u) = 2f(u, v) \quad \text{or} \quad f(v, u) = 4f(u, v)
   \]
   - For \(f(v, u) = 2f(u, v)\):
     \[
     5u^3 = 7u^2v + 49uv^2 + 13v^3
     \]
     Modulo 7, we get:
     \[
     \left(\frac{u}{v}\right)^3 \equiv -3 \pmod{7}
     \]
     which is impossible.
   - For \(f(v, u) = 4f(u, v)\), a similar contradiction arises.

8. **Conclusion:**
   Therefore, the only solution is:
   \[
   (a, b) = (2^i, 2^i) \quad \text{where} \quad i \geq 0
   \]

The final answer is \( \boxed{ (a, b) = (2^i, 2^i) \text{ where } i \geq 0 } \)