$CP$ violation is a necessary ingredient to create a matter-antimatter asymmetry. However, the amount of $CP$ violation incorporated in the Standard Model is orders of magnitude too small to explain the observed asymmetry. This thesis documents two studies of $CP$ violation with doubly charmed $B$ decays. Both were performed using Run I data collected by the LHCb experiment in $pp$ collisions at the LHC. This corresponds to 1.0 fb$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ collected at a centre-of-mass energy of 7 TeV and 2.0 fb$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ collected at a centre-of-mass energy of 8 TeV.

The first analysis is a search for $Bc+$ decays to two charm mesons, $Bc+\to D(s)(\ast )+D(\ast )$, where the second charm meson is either $D0$ or $D―0$, which are sensitive to the CKM angle \gamma. No evidence for signal is found and limits are set on twelve decay modes. The second analysis is the measurement of the $CP$ asymmetry in $B+\to D(s)+D―0$ decays. The results are \begin{align*} \mathcal{A}^{CP}(B^+\to D^+_{s} \overline{D}{}^0) &= (-0.4\pm 0.5\pm 0.5)%,\ \mathcal{A}^{CP}(B^+\to D^+ \overline{D}{}^0) &= (+2.3\pm 2.7\pm 0.4)%, \end{align*} where the first uncertainty is statistical and the second systematic. This is the first measurement of $ACP(B+\to Ds+D―0)$ and the most precise measurement of $ACP(B+\to D+D―0)$ to date. The results are consistent with $CP$ symmetry.