Position rack in middle of oven. Preheat oven to 350°F/180°C. Coat the inside of a 9-inch (23 cm) springform pan with nonstick spray and dust with 1 tablespoon of the cocoa. Tap out any excess cocoa and set prepared pan aside.

Melt the chocolate and butter together in the top of a double boiler or microwave, with gentle heat, until about three-quarters of the way melted. Remove from heat and gently stir until completely melted and combined and smooth. Whisk in 1/2 cup (99 g) sugar, vanilla, espresso powder, pinch of salt and remaining cocoa powder. Set aside until just warm to the touch in a large bowl (if it isn’t already).

Whisk egg yolks into butter/chocolate mixture until combined. Beat egg whites with an electric mixer on high speed in a clean, grease-free bowl until frothy. Add cream of tartar and continue beating until soft peaks form; gradually add 2 tablespoons of sugar and keep beating until stiff, but not dry, peaks form. Fold about one-quarter of the egg whites into the butter/chocolate mixture to begin to lighten it, then fold in remaining whites until batter is a uniform chocolaty color. Scrape batter into prepared pan.

Bake for about 20 to 30 minutes or until a toothpick shows many moist crumbs (more than you would usually see with a layer cake). The top of the cake will swell and most likely sport a crackly top crust, which is expected. Place on rack to cool for 10 minutes. Run a thin icing spatula around the outer edge of the cake, then release springform ring and remove. Cool cake completely. Serve in small slices, with a small dollop of unsweetened whipped cream or a few raspberries, if you like. Cake can be stored at room temperature overnight. You can refrigerate the cake for a couple of days and bring to room temperature before serving, but the texture will not be as delicate.