In saucepan melt butter; stir in brown sugar, corn syrup, and water.
Bring to boiling.
Add sweetened condensed milk; simmer, stirring constantly, till mixture reaches 230° (thread stage).
Transfer to fondue pot; keep warm over fondue burner.
If fondue becomes too thick, stir in a little milk.
Spear cake cubes on fondue forks; swirl in fondue to coat.