Line rimmed baking sheet with parchment paper. Spray parchment with cooking oil.
Heat Syrup
Combine sugar with water and syrup in large (preferably 5-quart), heavy-bottomed pot over medium heat. Stir until sugar is moistened.
Run damp pastry brush along edges of pot to remove any sugar crystals.
Bring to boil without stirring. As mixture begins to turn golden, gently swirl the pan. Cook until syrup turns golden and reaches 300°F on candy or digital thermometer, 3 to 4 minutes.
Make Sponge Toffee
Remove from heat and immediately whisk in baking soda just until combined. Mixture will bubble vigorously and expand.
Working quickly, scrape mixture onto parchment-lined tray and let rest untouched (don’t spread with spatula or toffee will deflate). Let rest until cool, 1 to 2 hours.
Crack And Dip Toffee
Crack sponge toffee into 1″ to 2″ shards and store in airtight container up to 5 days.
As needed, dip pieces into melted chocolate and transfer to a parchment paper-lined baking sheet. Let sit 1 to 2 hours or until chocolate has hardened.
