biblatex-ext's \DeclareOuterCiteDelims is usually implemented via the wrapper definition of the cite command. To wit \parencite is defined in the standard authoryear.cbx as
\DeclareCiteCommand{\parencite}[\mkbibparens]
{\usebibmacro{prenote}}%
{\usebibmacro{citeindex}%
\usebibmacro{cite}}
{\multicitedelim}
{\usebibmacro{postnote}}
but ext-authoryear.cbx has
\DeclareCiteCommand{\parencite}[\mkouterparencitedelims]
{\usebibmacro{prenote}}
{\usebibmacro{citeindex}%
\usebibmacro{cite}}
{\multicitedelim}
{\usebibmacro{postnote}}
So if you redefine \parencite you need to preserve \mkouterparencitedelims. Hence,
\DeclareCiteCommand{\parencite}[\mkbibparens]
{\usebibmacro{prenote}}
{\usebibmacro{citeindex}%
\printtext[bibhyperref]{\usebibmacro{citeyear}}}
{\multicitedelim}
{\usebibmacro{postnote}}
would have to be
\documentclass{memoir}
\usepackage{xcolor}
\usepackage[backend=biber, style=ext-authoryear]{biblatex}
\usepackage[colorlinks=true, urlcolor=blue, linkcolor=violet, citecolor=violet]{hyperref}
\DeclareOuterCiteDelims{parencite}{\bibopenbracket}{\bibclosebracket}
\DeclareCiteCommand{\parencite}[\mkouterparencitedelims]
{\usebibmacro{prenote}}
{\usebibmacro{citeindex}%
\printtext[bibhyperref]{\usebibmacro{citeyear}}}
{\multicitedelim}
{\usebibmacro{postnote}}
\addbibresource{biblatex-examples.bib}
\begin{document}
\parencite{sigfridsson}
\printbibliography
\end{document}
Something is a bit off here, though. That definition of \parencite only prints the year (\usebibmacro{citeyear}). It looks more like the definition of \parencite*. For the full citation you'd need \usebibmacro{cite}.
Note that you could run into trouble with double link targets if you just said
\DeclareCiteCommand{\parencite}[\mkouterparencitedelims]
{\usebibmacro{prenote}}
{\usebibmacro{citeindex}%
\printtext[bibhyperref]{\usebibmacro{cite}}}
{\multicitedelim}
{\usebibmacro{postnote}}
something like
\documentclass{memoir}
\usepackage{xcolor}
\usepackage[backend=biber, style=ext-authoryear]{biblatex}
\usepackage[colorlinks=true, urlcolor=blue, linkcolor=violet, citecolor=violet]{hyperref}
\DeclareOuterCiteDelims{parencite}{\bibopenbracket}{\bibclosebracket}
\DeclareFieldFormat{citehyperref}{%
\DeclareFieldAlias{bibhyperref}{noformat}% Avoid nested links
\bibhyperref{#1}}
\DeclareCiteCommand{\parencite}[\mkouterparencitedelims]
{\usebibmacro{prenote}}
{\usebibmacro{citeindex}%
\printtext[citehyperref]{\usebibmacro{cite}}}
{\multicitedelim}
{\usebibmacro{postnote}}
\addbibresource{biblatex-examples.bib}
\begin{document}
\parencite{sigfridsson}
\printbibliography
\end{document}
is safer in that regard (see hyperlink name with biblatex authoryear (biblatex 1.4b)).
![[Sigfridsson and Ryde 1998]](../../images/6346b00a0fb905b4b5fa540764093558.webp)
bibstyle=authoryear,style=ext-authoryearis redundant and equivalent tostyle=ext-authoryear.stylesets bothbibstyleandcitestyle, so the laterstyleoverwrites the earlierbibstyle.defernumbers=true,is generally not needed with non-numeric styles. – moewe Aug 03 '20 at 05:36\bibliographytakes the file name of the bibliography database without extension, so\bibliography{\jobname.bib}should be\bibliography{\jobname}. On modern systems it is no longer an issue to wrongly include the.bibextension, but until very recently MikTeX's BibTeX would not find the.bibfile in that case. Withbiblatex\addbibresourceis preferred over\bibliographyanyway. In\addbibresourceyou must include the file extension. So\addbibresource{\jobname.bib}would be even better. – moewe Aug 03 '20 at 05:39