Build a NAND gate using multiple NAND gates
Three gates.  Compute (((A nand A) nand B) nand B).  Unlike the four-gate solution above in which three of the four gates compute A nand B, two of them simultaneously, and substituting one of the simultaneous outputs for the other would yield two consecutive inverters, the three-gate solution uses the three gates to compute different functions: "not A", "A or not B", and "A nand B".  Only the last gate computes the desired function.