Lightly oil a 9x5-inch loaf pan and line it with parchment paper, leaving overhang on two sides.

Combine salted butter and heavy cream in a microwave-safe bowl; heat until melted and hot, about 1-2 minutes.

In a medium saucepan over medium heat, combine water, light corn syrup, and granulated sugar; stir until dissolved, then bring to a boil.

Cover the pan for about 1 minute to allow steam to form, helping to prevent sugar crystals.

Attach a candy thermometer and cook syrup without stirring until it reaches 320°F (8-10 minutes).

Carefully pour in the butter and cream mixture while stirring continuously to avoid overflow.

Cook mixture until it reaches 240°F (5-10 minutes), stirring occasionally, until thickened.

Pour caramel into the prepared loaf pan and cool for 20 minutes at room temperature.

Sprinkle flaked sea salt over the caramel and let it set for about 3.5 hours.

Once set, lift the slab out using the parchment overhang and cut into 1-inch by ½-inch rectangles.