Grease and line a baking sheet with parchment paper.
Combine sugar and milk in a large stainless steel pan; stir well. Add sweetened condensed milk and butter; bring to a boil. Reduce heat to medium-low and simmer, stirring often, until mixture turns a deep tan color and is thickened, about 10 minutes. Heat to 240 degrees F (115 degrees C) or until a small amount of mixture dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.
Vigorously stir mixture, scraping sides down and mixing well. Pour mixture onto prepared baking sheet. Allow to cool at room temperature until set, 1 to 3 hours. Cut into small squares or "tablets".
It should set relatively quickly (1 hour) but will retain a lot of latent heat until at least 3 hours after it is made. Hence be very careful trying the tablet before it is completely cold as you may well suffer a burnt tongue.
Per Serving: 247 calories;4.9 g fat;
50.9 g carbohydrates;
1.6 g protein;
15 mg cholesterol;
48 mg sodium.