Grease an 8 by 8-inch baking dish with butter. Line it with 2 pieces of parchment paper so that the paper hangs off all 4 sides, and then generously grease the paper with more butter.
Add the sugar, butter, corn syrup, and salt to a heavy-bottomed, deep-sided, medium-sized saucepan.
Heat over medium-high heat until the mixture turns amber colored, about 10 to 15 minutes. After cooking a couple minutes, carefully stir to help the sugar dissolve, but try not to get sugar crystals stuck to the sides of the pot. Once the sugar is dissolved, I recommend carefully swirling the pan (not stirring) to avoid sugar crystalizing on the sides. If you want to check the temperature, once the mixture is amber colored it should be 300F on a candy thermometer (hard-crack stage).
Once amber colored, remove it from the heat and use a wooden spoon to stir in the baking soda and vanilla. (Be careful because the mixture will foam up.) Stir until the ingredients are fully combined, but be don't over-stir or your sponge candy will deflate.
Pour the candy into the prepared dish and don’t move the dish until the candy is fully set, about 1 to 2 hours.
Once set, break or cut the candy into about 1 to 2-inch-sized pieces. (You will have smaller crumbs and shrapnel; it’s still delicious, so don’t discard it. Use it to garnish cupcakes, cake, ice cream, pancakes, waffles, yogurt, etc.)
Melt the chocolate in a microwave or double boiler, and temper it if desired. To coat the candy, gently brush off any small crumbs on the candy and dip it in the chocolate. Place it on a wire rack so any excess chocolate can drip off.
Let the chocolate set before serving.