Preheat oven to 300°. Grease bottom and sides of a 9" springform pan with butter. Line bottom with parchment paper; grease with butter. Dust paper with cocoa. Microwave bittersweet chocolate, 4 oz of the semisweet chocolate, and ¾ c butter in a microwavable bowl on MEDIUM (50% power) until melted and smooth, 2-3 minutes, stirring after 1 minute and then every 30 seconds. Cool 15 minutes.

Beat eggs, sugar, coffee, salt, and 2 tsp vanilla with a stand mixer fitted with whisk attachment on medium-high speed, until mixture is very foamy, pale in color, and doubled in volume, 3-4 minutes. Reduce mixer speed to low; gradually add melted chocolate mixture. Increase speed to medium-high, and beat until well blended, about 30 seconds. Sift cocoa over mixture; beat on medium-low speed just until blended, 20-30 seconds.

Pour batter into prepared pan; bake in preheated oven until a thermometer inserted in center of cake registers 140°, 40-45 minutes. Cool in pan on a wire rack 30 minutes.

Gently push edges of cake down with your fingertips until top is even. Run a knife around edges of cake; invert onto a wire rack. Remove pan and parchment paper. Cool 1 hour. Cover with plastic wrap. Chill 8 hours or up to overnight.

Microwave corn syrup and remaining 6 ounces semisweet chocolate and ¼ c butter in a microwavable bowl on MEDIUM until melted, 1-1½ minutes, stirring every 30 seconds. Stir in remaining ½ tsp vanilla.

Place wire rack with cake over a rimmed baking sheet. Pour glaze over cake, and spread over top and sides. Press chopped toasted pecans along sides. Chill cake 20 minutes before serving.