\documentclass[11pt, a4paper]{book}
\usepackage{fullpage}


\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}


\usepackage{graphicx}
\usepackage{xcolor}
\usepackage{geometry}
%%%%%%%%%%%%%%%%% CUP style %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage[explicit]{titlesec}
\usepackage{tikz}
\usepackage{lipsum}

\titleformat{\chapter}[block]
  {\normalfont\huge\bfseries}
  {\tikz\node[font=\huge\bfseries\color{white},fill=gray!50,rounded corners=20pt,minimum height=1.6cm,text width=3em,align=center,inner xsep=0pt] {\parbox{1.5em}{\thechapter\hfill}};}
  {-1em}
  {\tikz\node[fill=gray,font=\Large\sffamily\color{white},minimum height=1.6cm,text width=\the\dimexpr\textwidth-2em\relax,align=center,inner xsep=0pt] {#1};}
\titleformat{name=\chapter,numberless}[block]
  {\normalfont\huge\bfseries}
  {\tikz\node[font=\huge\bfseries\color{white},fill=gray!50,rounded corners=20pt,minimum height=1.6cm,text width=3em,align=center,inner xsep=0pt] {\parbox{1.5em}{\mbox{}\hfill}};}
  {-1em}
  {\tikz\node[fill=gray,font=\Large\sffamily\color{white},minimum height=1.6cm,text width=\the\dimexpr\textwidth-2em\relax,align=center,inner xsep=0pt] {#1};}

\begin{document}

\tableofcontents

\end{document}