In a saucepan, heat sugar over medium heat, stirring constantly. Sugar will form clumps and eventually melt into a thick brown, amber-colored liquid as you continue to stir. Be careful not to burn it.
Once sugar is completely melted, immediately add the butter. The caramel will bubble rapidly when the butter is added. Be careful. Stir the butter into the caramel until it is completely melted, about 2-3 minutes.
Very slowly, drizzle in the heavy cream while stirring. Allow the mixture to boil for 1 minute. It will rise in the pan as it boils. Remove from heat and stir in 1 teaspoon of salt. Transfer salted caramel into a jar or bowl. Let cool in the fridge to slightly solidify for about 10-15 minutes. You will be adding small blobs of salted caramel on top of the batter but it must be cooled first.
Preheat oven to 350 degrees F. Line an 8x8 pan with parchment paper and spray with cooking spray.
In a bowl, add butter, sugar, cocoa powder, eggs, flour, vanilla, and sea salt until everything is well combined. Add batter evenly onto the pan.
Now here’s where you will add the salted caramel, make sure it is slightly solidified enough where you can make swirls but not too liquid-y where it’ll just be a watery mess. Using a teaspoon, top the brownie batter with small blobs of salted caramel. Using a knife, gently make 3-4 swirls to give a marble effect.
Bake for 30-40 minutes, or until the toothpick comes out with moist crumbs. Do not overbake. Cool completely before serving. Drizzle more salted caramel on top, if desired, and sea salt. Enjoy!