Warm your pan: Use a medium, light-colored saucepan so you can see the sugar change color. Set it over medium heat.
Melt and caramelize the sugars: Add the granulated sugar and brown sugar.
Stir occasionally as they melt and clump. Keep stirring until the mixture turns smooth and a deep amber color, about 4–6 minutes. If it smokes, lower the heat.
Add the butter: Stir in the butter until fully melted and glossy.
The mixture will bubble—this is normal. Keep stirring.
Stream in cream: Slowly add the heavy cream while stirring. The caramel may seize a bit at first; keep stirring over low heat until smooth again.
Add milk and flavor: Pour in the milk, then add the vanilla and 1/4 teaspoon sea salt.
Whisk to combine and bring just to a gentle simmer. Do not boil hard.
Adjust thickness (optional): For a thicker creamer, whisk in the cornstarch slurry and simmer 30–60 seconds, stirring, until slightly thickened.
Taste and fine-tune: Add a pinch more salt if needed. You want a balanced sweet-salty taste, not a salty one.
Cool and store: Let the creamer cool 15–20 minutes, then transfer to a clean bottle or jar.
Seal and refrigerate.
Use: Shake before using. Start with 1–2 tablespoons per cup of coffee and adjust to taste.