An FPT algorithm for Hamiltonian cycle running parameterized by treewidth I'm looking for an algorithm that solves the Hamiltonian cycle problem parameterized by treewidth. In particular, I'm curious about such an algorithm running in $\text{tw}(G)^{O(\text{tw}(G))} \cdot n$ time.

In other words, once you have the treewidth, an algorithm that says if the input graph has a Hamiltonian cycle.
If you want an FPT algorithm for the problem (parameterized by treewidth $t$), you want an algorithm working in time $f(t) \cdot n^{O(1)}$, where $f$ is any computable function (depending solely on $t$). Of course, it would be nice to make $f$ as appealing as possible.

In addition to the mentioned algorithm running in $O(t^t n)$ time, you can also get a faster (randomized) algorithm using the Cut'n'Count technique of Cygan et al. [1]. In particular, you get an algorithm running in time $4^t n^{O(1)}$. It is also possible to get a deterministic algorithm working in $c^t n^{O(1)}$ (for some small constant $c$) using a rank-based approach of [2].



[1] Cygan, M., Nederlof, J., Pilipczuk, M., Pilipczuk, M., van Rooij, J. M., &amp; Wojtaszczyk, J. O. "Solving connectivity problems parameterized by treewidth in single exponential time." IEEE 52nd Annual Symposium on Foundations of Computer Science (FOCS), 2011.

[2] Bodlaender, H. L., Cygan, M., Kratsch, S., &amp; Nederlof, J. "Deterministic single exponential time algorithms for connectivity problems parameterized by treewidth." Automata, Languages, and Programming. Springer Berlin Heidelberg, 2013. 196-207.