Ensure the unsalted butter and heavy cream are at room temperature. Measure out all ingredients and have them ready.
In a heavy-bottomed saucepan, combine 1 cup granulated sugar, ¼ cup water, and ¼ teaspoon lemon juice. Stir gently to moisten the sugar, but do not stir once it begins heating.
Place the saucepan over medium-high heat. Allow the mixture to come to a boil without stirring. Watch closely as it cooks.
Continue heating until the sugar mixture turns a deep amber color. You can gently swirl the pan to move the caramel around and ensure even coloring, but avoid stirring. This process should take around 8-10 minutes. Do not let it darken too much, as it can taste burnt.
Once the caramel reaches the desired color, immediately remove the saucepan from the heat.
Carefully add 5 tablespoons of room-temperature unsalted butter to the caramel. It will boil and steam. Stir gently until fully incorporated. Then, slowly pour in ½ cup of room-temperature heavy cream, stirring continuously. The mixture will bubble up; keep stirring until smooth.
Stir in 1 teaspoon of vanilla extract, 1 teaspoon of sea salt, mix well.
Place in the refrigerator until cooled and slightly thickened.