Combine ingredients.In a large heavy-bottom stockpot, combine the milk, cream, sugar (or honey), and baking soda. Stir well to dissolve the sugar.
Begin the reduction.Place the pot over low heat. Slowly bring the mixture up to a gentle simmer, stirring occasionally to prevent scorching.
Cook slowly.Continue simmering the milk mixture over low heat, stirring regularly. As the milk reduces, it will gradually thicken and darken slightly in color.
Allow time for thickening.This process typically takes 1½–2 hours. Do not rush the heat — slow reduction is what creates the rich texture and flavor.
Check consistency.The mixture is finished when it has reduced significantly and coats the back of a spoon with a thick, glossy consistency.
Cool and store.Remove from heat and allow the condensed milk to cool slightly. Pour into clean glass jars or containers and refrigerate.