Step 1 In a medium, heavy-bottom pot, combine the milk, sugar and cinnamon over medium heat, stirring very gently with a wooden spoon. Once the sugar dissolves, stir in the baking soda and salt. Continue to cook, stirring very gently, until the mixture is thickened and creamy, about 1 hour. Do not allow the milk to boil over the pan (mostly because this will create a huge mess). When the mixture is thickened and resembles a caramel sauce, remove the pan from the heat; the dulce de leche is ready. This makes about 1 1/2 cups dulce de leche; more than is needed for the remainder of the recipe. The dulce de leche will keep, covered and refrigerated, up to 10 days.