Elemental boron is electron-deficient and cannot form graphene-like structures. Instead, triangular boron lattices with hexagonal vacancies have been predicted to be stable. A recent experimental and computational study showed that the B₃₆ cluster has a planar C_6v structure with a central hexagonal hole, providing the first experimental evidence for the viability of atom-thin boron sheets with hexagonal vacancies, dubbed borophene. Here we report a boron cluster with a double-hexagonal vacancy as a new and more flexible structural motif for borophene. Photoelectron spectrum of B₃₅^– displays a simple pattern with certain similarity to that of B₃₆^–. Global minimum searches find that both B₃₅^– and B₃₅ possess planar hexagonal structures, similar to that of B₃₆, except a missing interior B atom that creates a double-hexagonal vacancy. The closed-shell B₃₅^– is found to exhibit triple π aromaticity with 11 delocalized π bonds, analogous to benzo(g,h,i)perylene (C₂₂H₁₂). The B₃₅ cluster can be used to build atom-thin boron sheets with various hexagonal hole densities, providing further experimental evidence for the viability of borophene.