For the cake:
3 tablespoons unsalted butter, plus more for pan
6 ounces bittersweet chocolate, chopped
6 large eggs, separated, at room temperature
1 cup granulated sugar
3 tablespoons instant espresso powder
¼ teaspoon coarse salt
1 tablespoon vanilla extract
For the glaze:
3 ounces bittersweet chocolate, chopped
1½ tablespoons unsalted butter
2 teaspoons vanilla extract
⅓ cup heavy cream
⅓ cup granulated sugar
1 tablespoon instant espresso powder
¼ teaspoon coarse salt
Make the cake: Heat 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 ½ 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 ½ 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.