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 the room temperature unsalted butter to the caramel. It will boil and steam. Stir gently until fully incorporated. Then, slowly pour in 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.