A wealthy Brooklynite has recently inked a $27,000 per-month lease for an apartment in Downtown Brooklyn, marking the most expensive apartment in the borough’s history.  The unnamed renter signed the lease for the under-construction four-bedroom penthouse at One Boerum Place, a 96-apartment building just steps from Brooklyn Borough Hall. The eye-watering monthly rent falls just short of the most expensive lease overall, which belongs to a townhouse in Brooklyn Heights for $30,000 each month that was signed back in June. The mammoth lease was first reported by Crain’s New York Business on Thursday. The One Boerum Place penthouse features 3,100 square feet of indoor space and 2,000 square feet of outdoor space, according to a rep for One Boerum developer Avery Hall. The building …