Top padding inside tabular when using parbox

You can add \vspace{.5\baselineskip} soon after \parbox[t]{10cm}{. Adjust .5\baselineskip suitably.

\documentclass[10pt,a4paper]{article}
\usepackage{longtable}
\begin{document}
\begin{longtable}{| p{105 mm} |}
\hline
HEADER \\
\hline
\parbox[t]{10cm}{
\vspace{.5\baselineskip}  %% this line added
\rule{100mm}{50mm}
Fake caption
\strut}\\
\hline
\end{longtable}
\end{document}