Make the salted caramel

1. Heat the sugar in a medium saucepan over medium heat until fully melted and amber.
2. Add the butter and stir until smooth.
3. Pour in the heavy cream in 2–3 additions, stirring constantly.
4. Cook for 3–4 minutes until slightly thickened.
5. Stir in the fleur de sel.
6. Blend briefly with an immersion blender if needed.
7. Transfer to a bowl or jar and let cool completely.

Make the Swiss meringue buttercream

1. Combine the egg whites and granulated sugar in a heatproof bowl.
2. Heat over a double boiler, whisking constantly, until the mixture reaches 160°F (71°C) and the sugar is fully dissolved.
3. Whip in a stand mixer fitted with the whisk attachment on medium-high speed until stiff, glossy peaks form and the bowl feels completely cool.
4. Add the softened butter gradually on medium speed, then whip until smooth and silky (the mixture may look curdled at first—keep mixing).
5. Mix in the vanilla extract and the cooled salted caramel until fully incorporated.
6. Finish by mixing on low speed for 1–2 minutes to remove air bubbles and smooth the texture.