• Place the evaporated milk and the condensed milk in a heavy saucepan. Add the baking soda, and salt.
• Cook and stir constantly (mixture will scorch easily, so don't walk away!) over medium low heat (adjust heat so that the mixture stays just barely at a simmer). Cook and stir until the mixture is very thick, about 30 to 45 minutes. (To test: Lift the spoon out of the pot and drizzle some of the caramel over the surface. If it forms a ribbon that does not disappear after 10 seconds or so, it is ready. Also check by dragging the spoon along the bottom of the pot. You should be able to see the bottom of the pot for a few seconds before the thickened mixture closes in on itself and covers the bottom.)
• Remove from heat and let cool for 5 minutes. Stir in the vanilla and corn syrup.
• Let cool completely. Store, covered, in refrigerator.

Alternative

• Place the can on its side in a large pot. Fill the pot with room-temperature water, making sure the water level is at least 2 inches above the can.
  
• Set pot over high heat and allow to come to a simmer. Reduce heat and simmer for 2 hours for a lighter caramel, and up to 3 hours for a darker caramel; check the pot every 30 minutes to ensure the water level stays above the can, adding boiling water as necessary to top it up.
• Using a pair of tongs, remove the can from the water and set on a wire rack to cool to room temperature (important: do not attempt to open the can while still hot, which can cause pressurized hot caramel to spray dangerously).
• Unopened cans of dulce de leche can be stored at room temperature for up to 3 months. To use dulce de leche, open can when cool and scoop out of can; reheat in a double boiler to soften dulce de leche to a spreadable or drizzle-able consistency. Dulce de leche can be tranferred to an airtight container and refrigerated for up to 3 weeks.