400 g granulated sugar
400 ml water + 1 oz
½ tsp Kosher salt
Add a small amount of water (about 1 oz) to the bottom of a pot. Pour 400 grams of granulated sugar over the water and heat over medium, stirring occasionally, until the mixture turns a deep amber color and starts to smell almost (but not quite) burned. Immediately add 400 ml of water (it will boil and sizzle violently, which is why I use a pot for this) and remove from heat. Stir the mixture until everything is dissolved, then add ½ tsp of kosher salt. Stir until salt is dissolved, then let cool and store in fridge.
*Adapted from Dave Arnold's Liquid Intelligence