Line an 8-inch square pan (11×7 pan would work too) with parchment paper.
Combine sugar, water, corn syrup, and cream of tartar in a heavy bottomed saucepan and heat over high heat. Stir occasionally until sugar has dissolved. Cover and continue to cook for 5 minutes. Combine heavy cream and soy sauce in a liquid measuring cup and set aside. Cut butter into 8 pieces and set aside.
Remove lid from the sugar mixture and attach your candy thermometer to the side of your pan. As soon as the sugar mixture reaches 230F, reduce heat to medium and cook, without stirring, until the syrup is golden in color and starts approaching 300F (6 to 7 minutes). Swirl the pan gently to ensure even heat. When the temperature approaches 350F (the sugar will start looking amber in color). Remove the pan from the heat gently swirl again.
Cool for two minutes.
Pour the cream and soy mixture and butter into the pan (Carefully! It will bubble!). Stir to combine. Return the caramel to medium heat and stir until the butter is completely melted.
Continue cooking until caramel reaches 255F. Remove from the heat and pour into the parchment-lined pan, tap gently on counter release air bubbles. Place pan onto cooling rack and cool for 30 minutes. Sprinkle evenly with sea salt and continue cooling on the rack for an additional 3 1/2 hours. Cut into 1-inch pieces and wrap individually in parchment.
