Step 1

Spray the mold (see notes) with the oil.

Step 2

In a large pot or 3-quart saucepan, heat evaporated milk over medium-low heat. Add the sugar and lime rind.Once it breaks the boil, adjust the flame until it settles on a gentle boil, and simmer, stirring with a wooden spoon or spatula as necessary to prevent it from overboiling or burning.Once it starts thickening (30-40 mins), whisk vigorously until it has become so thick that it sticks to the spoon (similar consistency to peanut butter).

Step 3

Discard the lime rind, spoon the mixture into the prepared mold, and smooth it out. Let it cool to room temperature.Remove from the mold and cut into six squares.