Piment d'Espelette is a chile pepper that comes from Espelette, a Basque enclave near the Spanish border, but is popular throughout France. If you can't find piment d'Espelette, cayenne is a great alternative. Don't worry if the center of the cake is still molten after baking; that's how it's supposed to be. Vanilla ice cream is a delicious accompaniment to this chocolate cake and helps offset its spiciness.
Preheat the oven to 350 degrees. Bring a saucepan of water to a boil, then lower to a simmer. Cut the butter and chocolate into small pieces and place with the piment d'Espelette in a metal bowl large enough to sit on top of the saucepan. When the butter and chocolate have melted completely, set aside and stir until well combined. Let cool slightly.
Meanwhile, separate the egg yolks from the whites. Whisk the egg yolks together with the sugar, then add the chocolate mixture.
Using a mixer, beat the egg whites and salt until stiff peaks form. Fold half of the egg whites into the chocolate batter, then fold in the remaining egg whites. Let sit in a refrigerator for one hour.
Grease a cake pan with butter, then pour the chocolate batter into the pan. Bake for about 20-25 minutes or until cracks start to form on the top of the cake. Serve hot.