White butts means the top of the canelés are white when they come out of the mold. This usually happens when the canelés mushroom out of their mold and do not deflate. The middles are usually pretty hollow when this happens. Do not overfill the molds. If it expands out of the molds without deflating later on, the pastry doesn't touch the top of the mold again. Without the high heat from the mold, the canelé doesn't caramelize on the top, resulting in "white butts." If there are white butts, they can be popped back into their molds and go back into the oven.