Add all ingredients to a sauce pan and simmer over medium heat occasionally stirring. The whole process takes approximately 2 to 2 1/2 hours. Keep an eye on the sauce pan as the mixture can sometimes foam up and spill over the edge. As the milk and sugar start to caramelise stir more often to avoid sticking or burning.
As the mixture gets thicker you can perform a simple test on a cold ceramic plate. Put a dab of dulce de leche on the plate and turn sideways, it shouldn't run for more than 1-2 centimetres. If its still watery or runs much further cook another 10 minutes and repeat the test until ready.
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