Flourless chocolate cake is a crumbly and dense classic everyone loves. This version gets subtle coffee flavor from espresso powder used in the batter and in a rich, creamy dark-chocolate glaze served at the table.
Make the cake: Preheat oven to 350 degrees. Butter bottom of a 9-inch springform pan, and line with parchment cut to fit. Melt butter and chocolate in a heatproof bowl set over a pan of simmering water.
With a mixer fitted with the whisk attachment, beat egg yolks with 1/2 cup sugar until thick and pale, about 3 minutes. Add espresso and salt, and beat for 1 minute. Add vanilla and chocolate mixture, and beat for 1 minute.
In a clean bowl and with a whisk attachment, beat egg whites until foamy. Slowly add remaining 1/2 cup sugar, beating until stiff peaks form. Fold whites into chocolate mixture in 3 additions. Pour batter into prepared pan.
Bake until set, 40 to 45 minutes. Let cool completely in pan on a rack. Remove pan sides. Carefully lift cake with a spatula, and remove parchment.
Make the glaze: Place chocolate, butter, and vanilla in a bowl. Bring remaining ingredients to a boil, stirring, and pour over chocolate mixture. Whisk until smooth. Serve glaze warm with cake.