Prepare the Caramel:

1. Make the Caramel Sauce: In a small saucepan, melt the sugar over medium heat, stirring occasionally until it turns into a golden caramel. Once the sugar is melted, carefully add 1 tablespoon of butter, and stir until it’s fully incorporated.
2. Add Heavy Cream: Slowly pour in the heavy cream while stirring. Be cautious as the mixture may bubble up. Stir until smooth.
3. Add Sea Salt: Remove from heat and stir in a pinch of sea salt. Set aside to cool while preparing the mousse.

Make the Chocolate Mousse:

4. Melt the Chocolate: In a heatproof bowl, melt the dark chocolate and 2 tablespoons of butter together. You can melt it in the microwave in 20-second intervals or over a double boiler until smooth.
5. Whisk the Cream: In a separate bowl, whip the heavy cream until soft peaks form. This will take about 2-3 minutes with an electric mixer.
6. Combine: Gently fold the whipped cream into the melted chocolate, adding in the vanilla extract. Be careful not to deflate the whipped cream, as it gives the mousse its light, airy texture.

Assemble the Mousse Cups:

7. Layer the Mousse and Caramel: Spoon a layer of chocolate mousse into the bottom of individual cups, followed by a layer of salted caramel sauce. Repeat the layers, finishing with a generous dollop of mousse.
8. Chill: Refrigerate the mousse cups for at least 2 hours, or until set and chilled.