This dense moist cake is like a giant chocolate truffle—crunchy on top with a deliciously fudgy center. Extremely rich yet so simple to make, it is popular all year round. It is gluten-free and you can even make it in advance—just take it out of the refrigerator an hour before serving. It tastes even better the second day.
Preheat oven to 350°F (177°C). Grease an 8-inch springform pan with a small amount of butter. Melt chocolate in a microwave on high in 30-second intervals, stirring in between, until chocolate is fully melted and smooth; set aside. Combine cocoa, baking powder and salt in a medium bowl. In a separate bowl, beat together butter and sugar; add eggs one at a time, beating well after each addition, and continue beating until mixture is pale and creamy, about 5 minutes. Beat in vanilla. Add chocolate; beat until just blended. Fold in ground walnuts, chopped walnuts and cocoa mixture. Pour into prepared pan and bake until cooked through, about 35 minutes. Let cool completely on a wire rack. Run a small knife around edges of springform pan before releasing cake. Serve at room temperature, topping with one or more garnishes to taste.