Rapid, continuous flickering is a characteristic feature of compact, accreting sources, mostly seen in X-rays. In one of the fastest multi-wavelength timing studies of black hole X-ray binaries to date, we have discovered correlated optical and X-ray variability in the low/hard state of two sources: GX 339-4 and SWIFT J1753.5-0127. The observations were carried out simultaneously with VLT/Ultracam and RXTE, and the data reveal intriguing correlation patterns between the two bands down to the shortest time resolution available (~50 ms). Usual reprocessing models can be ruled out as the origin of the aperiodic optical power. Instead, the variability may be driven by synchrotron emission from the inner accretion flow regions, with interactions between the disk, jet and corona resulting in complex correlation patterns. Observations probing very short timescales are allowing us to place important constraints on the geometry of accreting compact sources and, more generally, are giving us new insight into the behaviour of a variety of hot astrophysical plasmas.