Set a medium, heavy-bottomed saucepan over medium-low heat. Add the sweetened condensed milk, whole milk, brown sugar or panela, and baking soda. Whisk until smooth.

Bring the mixture to a gentle simmer, stirring often with a heatproof spatula or wooden spoon. Scrape the bottom and corners of the pan each time you stir so the milk does not stick or scorch.

Lower the heat to keep the mixture at a very gentle simmer. Continue cooking and stirring frequently for 25 to 35 minutes, until it is a warm caramel color, noticeably thicker, and leaves a clear trail for 1 to 2 seconds when the spatula is dragged across the bottom of the pan.

Remove the pan from the heat. Stir in the butter, vanilla, and salt until the arequipe is glossy and completely smooth.

For a silky texture, let the arequipe cool for 10 minutes, then transfer it to a heat-safe jar or bowl. It will thicken further as it cools. Serve warm, at room temperature, or chilled.