Step 1

Adjust oven rack to middle position and preheat oven to 350&degF. Butter 9-inch springform pan. In food processor, pulse 1 cup sugar with hazelnuts until finely ground.

Step 2

In large bowl, whisk yolks, hazelnut mixture, zest, butter, salt, and vanilla until combined; set aside.

Step 3

Using mixer fitted with whip attachment, beat whites on medium-high speed to opaque. Slowly add remaining 2 tablespoons sugar and continue to beat to soft peaks. Fold meringue into hazelnut mixture, alternating with panko crumbs.

Step 4

Pour into pan and bake until center is just set and toothpick inserted into the middle comes out clean (see headnote), about 40 minutes. Let cake cool in pan for 15 minutes, then remove sides of pan and let cake cool completely before glazing.

Step 5

In heatproof bowl, heat chocolate with cream and espresso powder, until chocolate is melted and the mixture is smooth. Place cake on wire rack set in a rimmed baking sheet. Pour ganache over cake. Chill to set ganache. Serve.