Combine sugar, corn syrup, and water in a 2 quart pot (you need a much bigger pot because it will expand). Bring to a simmer and cook, stirring frequently.
Once sugar is dissolved and mixture is bubbling, turn heat down to low and let simmer, without stirring, until mixture turns a dark amber color, about 8-10 minutes.
Once sugar is the dark amber color, immediately remove from heat and carefully stir in warmed cream. It will bubble and fizz like crazy.
Stir until smooth, scraping up any bits on the bottom of the pan. Add in butter and stir until smooth.
Stir in pinch of salt and vanilla. If mixture has any chunks in it at all, return to heat briefly (low heat) and stir until the mixture smooths out.
Cool and set aside until needed.
For Chocolate Cake:
In a large bowl, combine flour, sugar and salt.
In a saucepan, melt together butter and cocoa powder. Once smooth, stir in boiling water.
Add cocoa powder mixture to dry ingredients and stir to combine.
In a separate bowl, whip together eggs, buttermilk, baking soda, and vanilla.
Stir buttermilk mixture into batter. Try not to overmix it but make sure all the flour is mixed in.
Prepare your baking pan by buttering it lightly and dusting it with cocoa powder.
Add batter to 9x13 baking pan and bake at 350 °F for 30-35 minutes until a tester comes out clean in the center of the cake.
Finishing the Cake:
When cake comes out of the oven, poke a bunch of holes in it with a chopstick.
Mix the sweetened condensed milk with the caramel sauce and pour the mixture over the cake. Use a spatula to scrape the sauce back and forth so the sauce gets completely absorbed by the cake.
Let the cake sit at room temperature for 30 minutes.
Chill cake for at least an hour, but preferably a few hours.
Whip cream with sugar and vanilla until it forms stiff peaks and then frost cake with fresh whipped cream. Top with crumbled heath bars or chocolate chips!