Place sugar and whole milk into a large saucepan. Stir to combine.Add condensed milk, corn syrup, and butter.
Heat over medium-high, stirring occasionally. Bring to a boil.
Reduce heat and continue to boil gently for approx 20-30mins, stirring often, until mixture has thickened and turned a light caramel color (and 240F on a candy thermometer).
Remove from heat. Using a hand mixer on low speed (be careful as the mixture is HOT!!!), beat the mixture until it thickens and loses its shine (5-7minutes).*
Pour into sheet pan (ensure it is on a heat proof surface) and allow to cool. When set but still warm, score the tablet into squares with a sharp knife.
Once cool to the touch, place in fridge to set overnight. Cut into squares with a serrated knife. Store in the refrigerator in an airtight container.**
Notes
* You can do this by hand, and stir vigorously with a wooden spoon, but it will take a bit longer. ** You can eat it straight out of the fridge or let it come to room temperature. Note that it will be very soft and gooey inside at room temperature. Adapted from Scottish Butter Tablet and Moje Wypieki