Preheat oven to 300 degrees. Coat an 8-inch round cake pan with nonstick spray, then line it with parchment paper (I trace the outside of the pan on parchment and cut it out). Coat the parchment with nonstick spray, then dust with coca powder.
Set a glass or metal bowl over a pot of gently simmering water to create a double boiler (do not let the water touch the bowl). To the bowl, add chocolate and butter to melt, whisking until smooth and blended. Set aside and cool slightly.
In a standing mixer fit with the paddle attachment on medium-high speed, or with an electric hand mixer, add together egg yolks, 1/4 cup sugar, espresso or rum (if using), vanilla, and salt. Beat until pale and very thick.
Gradually pour in the melted chocolate mixture and beat until blended. Transfer to a large bowl and wash out the mixer bowl so you can beat the egg whites.
In a standing mixer fit with the whisk attachment on medium-high speed, or with an electric hand mixer, beat egg whites until foamy. Add remaining 2 tablespoons sugar and continue to beat until medium-firm peaks form.
Scoop half the egg whites into the chocolate mixture and fold in gently. Fold in remaining egg whites until no streaks remain. Pour batter into prepared pan and smooth the top.
Bake until the cake puffs slightly and a toothpick inserted into the center comes out very moist but not liquid, about 35 minutes. Do not over-bake. Cool on a rack for at least 30 minutes.
Run a small knife around the inside of the pan to loosen the cake, then invert on to a plate. Lift off the pan and carefully peel off parchment paper. Cool completely, then cover and refrigerate until very cold, at least 4 hours or overnight.