Fix a candle that burns down the center by resetting the wax pool so it melts evenly.
Fold the foil loosely over the edges so it creates a dome — this traps heat and forces the outer wax to melt.
Check it periodically. Once the melt pool reaches the edges, the tunnel is reset.
Going forward, always burn the candle long enough for the wax to pool edge-to-edge on the first use — typically 1 hour per inch of diameter.