Preheat oven to 325 F. Grease a 10-inch spring form pan with cooking spray or butter. Line the bottom of the pan with a piece of parchment cut to fit exactly. Lightly grease the parchment.

In a medium sized saucepan, melt the butter, over medium-high heat, until bubbling. Reduce heat to low and add finely chopped chocolate, stirring until melted. Remove from heat and let sit while you whip the eggs.

In the bowl of a stand mixer with the whip attachment, or in a large bowl with a hand blender, whip the yolks with half the sugar (you can eye-ball it) until light yellow, fluffy and tripled in volume. Using a rubber spatula, fold the chocolate mixture, espresso powder and vanilla into the egg yolks until almost fully combined. Leave some streaks because you will be folding in the whites next.

In a clean bowl, whip the egg whites on medium speed until a layer of foam forms on the top. Slowly add the rest of the sugar and salt and continue beating until medium-stiff, glossy peaks form.

Stir one-third of the whites into the chocolate-yolk mixture to lighten it, then gently fold in the remaining whites just until no white streaks are visible. Immediately pour batter into the prepared pan and bake for 30 to 40 minutes or until the cake has puffed up and is no longer shiny on top. The cake should be set but will jiggle slightly.

Remove from oven and let cool completely in the pan on a wire rack. Run a small knife or metal palate knife gently around the edges to release it from the sides of the pan. Release and lift off the pan sides.

Serve at room temperature for a light, almost mousse-like texture. For a denser, fudgier cake, refrigerate at least 3 hours before serving. Garnish with a dusting of cocoa powder or icing sugar and a dollop of whipped cream if desired. Fresh berries are also a nice addition.