Add the condensed milk, milk, sugar, and butter to a saucepan. Heat over low heat and stir until everything melts together and the mixture becomes smooth and creamy.

Bring to a soft boil and simmer for 10 minutes, stirring constantly to prevent burning. Scrape the bottom of the pan regularly.

Check that the mixture reaches 240–245ºF (115–120ºC) using a thermometer.

If you don’t have a thermometer, drop a small spoonful into a bowl of ice water. If it forms a soft ball that doesn’t stick to your fingers, it’s ready. If not, continue simmering for a few more minutes and test again.

Remove from the heat and let it cool for a few minutes.

Whisk the mixture using an electric mixer or wooden spoon until it thickens, turns matte, and begins to set. This may take up to 10 minutes.

Pour the mixture into a parchment-lined 8-inch (20 cm) tray and smooth the top. Cover with another sheet of parchment. Let it cool to room temperature, then transfer to the fridge to chill until fully set.

Slice into 1–1.5 inch squares and enjoy or store in an airtight container.