For Caramel Sauce • ½ cup light brown sugar firmly packed (100g) • ½ cup dark brown sugar packed  (100g) • ½ cup unsalted butter cut into 8 pieces  • ¼ cup heavy cream • ¼ cup whole milk • 2 Tablespoons light corn syrup • ¼ teaspoon salt For Frosting • ¾ cup unsalted butter softened (170g) • 4 cups powdered sugar (500g) • 1 teaspoon vanilla extract • Fine sea salt for sprinkling

Combine brown sugars, butter, cream, milk, corn syrup, and salt in a medium-sized saucepan over medium-low heat. Cook, stirring frequently, until the butter is melted and sugar has dissolved.

Increase heat to medium and bring mixture to a full boil, stirring constantly.

Once mixture reaches a full boil, set a timer for 4 minutes and stir occasionally while caramel cooks (or use a candy thermometer and cook until the sauce reaches 230F/110C).

Remove from heat and pour into a large heatproof bowl. Allow caramel to cool completely before proceeding.

Place softened butter in a large mixing bowl and use an electric mixer to beat until smooth and creamy.

With mixer on low-speed, gradually add about half of the powdered sugar until combined. Drizzle in caramel and stir until completely incorporated. 

Slowly adding remaining powder sugar to caramel frosting or until desired consistency is reached, stirring well in between each addition.

Stir in vanilla extract.

Spread or pipe over cake or cupcakes, sprinkle with fine sea salt if desired.