2/3 cup (4 ounces) semisweet or bittersweet chocolate, chopped (Scharffen Berger 62%)
1/2 cup (1 stick/4 ounces) butter
3 large eggs, one separated
1 cup granulated sugar
1 teaspoon vanilla (Nielsen-Massey)
2 to 2-1/4 cups all-purpose flour
1/2 cup unsweetened cocoa (Bensdorp)
3/4 cup coarsely chopped roasted hazelnuts
1-1/2 teaspoons baking powder
1 teaspoon salt
1. Heat oven to 350 degrees. Melt the butter and chocolate together either in a double boiler or in a microwave. If using a microwave, microwave the chocolate alone until partially melted, scrape down the bowl and add the butter. Then heat for another 15 seconds, scrape down and check, heat for another 15 seconds if necessary to melt both the butter and the chocolate.
2. Beat the two whole eggs and one egg yolk and the sugar until lightened, about 2 minutes with a hand mixer.
3. Add vanilla and chocolate mixture.
4. Mix in the flour, cocoa, hazelnuts, baking powder, and salt just until combined. You should have a soft, but not sticky, dough. Add the extra 1/4 cup of flour if dough is too sticky.
5. Divide the dough in half. On a lightly floured surface, form each half into a log that is about 4 inches by 8 inches. Place logs on parchment paper on a large baking sheet and brush with lightly beaten egg white if desired. Bake for 20-25 minutes or until tops are set.
6. Reduce oven to 275 degrees. Let logs cool about 20 minutes and then slice into 1/2-inch thick slices. Arrange the slices on a rack on a baking sheet and bake for 20 minutes. Cool on a wire rack. Store in an airtight container or freeze.