Element, 10 bytes

Output:

a(3) = 6561 a(4) = 4294967296 

There's probably only a few ways to compute this sequence in Element. I found a 9-char solution, but I figured this 10-char solution is actually more difficult. The # are hidden characters.

#_####@##` 

For convenience, here's the Try It Online and Esolang wiki pages.