Chocolate Birthday Cake (Belgian)

This luscious chocolate-hazelnut cake is coated with a ganache made from chocolate and cream. It's topped with a mountain of bittersweet chocolate curls and surrounded by crunchy praline hazelnuts coated in tempered chocolate and rolled in cocoa powder.

4 oz hazelnuts
13⅓ Tbsp. butter (1⅔ sticks), softened
⅔ cup Dutch-process cocoa powder
6 Tbsp. granulated sugar
1⅔ cup all-purpose flour, unbleached
1½ tsp. baking soda
¼ tsp. sea salt
⅔ cup water, boiling
1¾ cups dark-brown sugar, packed
4 large eggs, at room temperature
1⅓ cups buttermilk, at room temperature
2½ tsp. pure vanilla extract

Decoration:
Chocolate Ganache
Candied Hazelnuts
Chocolate Curls

Preheat oven to 350°F (180°C/160°C fan, Gas Mark 4). Butter a 9x3" springform pan well; coat with cocoa, tapping out any excess.

Place nuts on a baking sheet. Bake until fragrant and toasted, about 12 minutes. Transfer to a clean kitchen towel; rub to loosen skins. 

In a food processor, grind nuts with granulated sugar until fine but not pasty; transfer to a large bowl; stir in the flour, baking soda, and salt.

In a heat-proof bowl, whisk together cocoa and boiling water until smooth. (Mixture will thicken as it cools.)

In the bowl of an electric mixer, cream butter and brown sugar on high until lightened and fluffy, 3-4 minutes. Beat in eggs one at a time until well blended.

Stir buttermilk and vanilla into cocoa mixture. Mixing on low, add half of dry ingredients to creamed mixture; when blended, pour in the cocoa mixture, and add remaining dry mixture, mixing just until incorporated. Scrape batter into prepared pan, smooth top. Bake 1 hour to 1 hour and 10 minutes, until a tester inserted in the center comes out clean. Let cool in pan for 10 minutes; remove from pan and cool completely. Place cooled cake on a 9" cardboard round.

Place half the ganache in a bowl; place in a larger bowl of ice water; whip with a balloon whisk until lighter in color and spreadable, removing bowl from ice bath and returning it as necessary.

Spread the whipped ganache smoothly on top and sides of cooled cake, and chill the cake. Gently stir remaining ganache every 5 minutes until thickened and cool.

Place cake (still on cardboard round) on a wire rack over a sheet of waxed or parchment paper. Working with a small ladle, pour ganache over top of cake, moving the ladle in a circular fashion while in contact with the cake, letting the ganache run down sides. Scrape up excess and reserve for another use.

Let cake stand at room temperature until set. Garnish with nuts and chocolate curls.

Makes 10 servings.

Cook's Notes:

• Toast and peel the nuts for the cake and garnish at the same time.
• Bake the cake up to three days ahead; wrap well and keep at room temperature.
• The candied hazelnuts and chocolate curls can also be made three days ahead; store in airtight containers.