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

Once the milk mixture comes to a gentle simmer, temporarily remove the saucepan from the heat. Carefully whisk in the baking soda, which will cause the hot liquid to foam and bubble up significantly.

Return the pan to low heat and stir in the vanilla extract and flaky sea salt. Simmer the mixture gently, stirring occasionally, for about 60 to 70 minutes as it slowly reduces and turns a golden color.

As the mixture darkens to a light caramel color, begin to stir more frequently to prevent the sugars from burning on the bottom of the pan. Continue cooking for another 15 to 20 minutes until it becomes deep amber and thickens to a pudding-like consistency.

Remove from heat once the dulce de leche coats the back of a spoon and leaves a clear path when you swipe your finger through it. Let it cool in the pan for 10 minutes before transferring it to a glass jar, and serve warm drizzled over your favorite desserts.