Combine the whole milk, granulated sugar, and kosher salt in a large, heavy-bottomed saucepan over medium heat. Stir continuously with a wooden spoon or heat-proof spatula until the sugar is completely dissolved.

Once the milk begins to gently simmer, briefly remove the saucepan from the heat and whisk in the baking soda. The mixture will foam up significantly, which is a normal reaction that helps prevent the milk proteins from coagulating.

Return the saucepan to medium-low heat and maintain a very gentle, steady simmer. Cook uncovered, stirring occasionally for the first hour, making sure to scrape the bottom and sides of the pan to prevent scorching.

After about an hour, the mixture will begin to darken into a light amber color and start to visibly thicken. At this crucial stage, you must stir much more frequently to prevent the milk sugars from burning at the bottom.

Continue to simmer and stir constantly for another 15 to 30 minutes until the mixture turns a deep, rich caramel color and coats the back of a spoon thickly. If you draw your finger through the sauce on the back of the spoon, it should leave a clear, distinct path.

Remove the pan from the heat and carefully stir in the pure vanilla extract. Transfer the hot dulce de leche to a heat-proof glass jar and let it cool completely at room temperature, where it will thicken into a perfect spread for toast or sweet treats.