Prepare two baking sheets lined with silicone baking mats or good parchment paper.
Combine butter, sugar, water, and salt in a heavy bottomed saucepan. Bring to a boil, stirring constantly.
Cook over moderate heat, stirring gently, until candy thermometer registers exactly 298℉.
Remove from heat and immediately stir in vanilla until well combined.
Working quickly, pour ½ the mixture onto each prepared baking sheet. Use a heatproof rubber spatula or offset spatula to spread quickly before the toffee sets. Spread thin.
Allow toffee to cool completely. Blot to remove excess oil from the surface.
Melt chocolate in a double boiler.
Pour ½ of melted chocolate over each piece of toffee, spread, and immediately sprinkle with topping.
Allow to set, then break into bite-size pieces.
RECIPE TIPS
VariationsHazelnut Toffee: Add in 1 cup of toasted and chopped hazelnuts to the toffee before you pour it. Keto: I have made this recipe with Lakanto monk fruit sweetener. I had really good success with this variation. I let the sweetener get a touch hotter, 300℉. How to storeStore in an airtight container. Add layers of waxed paper between the toffee if you will be keeping it for awhile.