We study the topologically twisted index of N ¼ 6 supersymmetric Chern-Simons matter theory with UðNÞ k × UðNÞ −k gauge group in the 't Hooft limit, that is, for N; k → ∞ with λ ¼ N=k fixed. In the regime where λ is fixed and large we find an analytical expression for the leading order term of the index. The leading term of the index matches precisely the Bekenstein-Hawking entropy of the dual asymptotically AdS 4 magnetically charged black holes embedded in IIA supergrvity on AdS 4 × CP 3 , after a standard Legendre transformation. We numerically explore the genus expansion of the topologically twisteed index beyond the leading order, focusing on the genus one contribution, that is, N 0. We find qualitative agreement with the topological expansion of the free energy on S 3 at genus one. Our logarithmic in λ term constitutes a prediction for the one-loop effective action on the IIA supergravity side.