Step 1

Adjust oven rack to middle position and preheat oven to 350°F. Line 9-inch springform pan with parchment and spray with nonstick pan spray.

Step 2

Sift flour, cocoa, and baking soda into small bowl; set aside. In medium bowl, whisk brown sugar, sour cream, oil, egg, espresso powder, 1 teaspoon vanilla, and salt until combined. Whisk in dry ingredients until smooth.

Step 3

Spoon batter into prepared pan and bake until just firm and a toothpick inserted into center comes out clean, 17 to 19 minutes. Transfer pan to wire rack to cool 20 minutes, then remove cake from pan to cool completely, about 1 hour.

Step 4

In small bowl, whisk granulated sugar with yolks. In medium saucepan over medium heat, bring 1/2 cup cream to simmer. Slowly whisk hot cream into egg mixture, then return entire mixture to saucepan. Cook over medium-low heat, stirring, until mixture just thickens, about 1 minute. Whisk in 1 teaspoon vanilla and 12 ounces bittersweet chocolate until smooth. Transfer to large bowl and let cool to room temperature, about 45 minutes.

Step 5

Using stand or hand mixer fitted with whip attachment, whip 1 1/2 cups cream to medium peak. Fold 1/3 of whipped cream into cooled chocolate mixture until combined and smooth. Fold in remaining whipped cream just until no white streaks remain.

Step 6

Line bottom and sides of springform pan with plastic wrap. When cake is cool, slice into two layers. Place one layer into bottom of pan. Fill with mousse and smooth top. Place second layer on top and gently press to adhere. Cover and chill until set, at least 3 hours.

Step 7

Place remaining 8 ounces bittersweet chocolate in medium bowl. In small saucepan, heat remaining 3/4 cup plus 2 tablespoons cream until simmering. Pour over chocolate and let sit 5 minutes. Whisk until smooth.

Step 8

Remove sides of springform pan and invert cake onto large plate. Peel off plastic wrap and replace bottom of springform pan. Invert onto wire rack and set rack into rimmed baking sheet. Pour ganache over cake, allowing it to drip over sides. Chill until set, about 30 minutes. Using offset spatula, spread any remaining ganache around the sides of cake.

Step 9

Melt semi-sweet chocolate in microwave or in pan over bowl of barely simmering water. Spread chocolate onto back of flat baking sheet (chocolate should be spread very thin). Chill briefly until just set.

Step 10

Holding bench scraper at 45 degree angle, scrape 2 to 3-inch wide strips of chocolate until it forms a curl. (If chocolate is breaking, it is too cold; let itwarm up slightly). Place curls on cake and dust with confectioners' sugar. See note for serving instructions.