Chocolate-Nut Cake:

1. Preheat the oven to 180 degrees Celsius and grease a 20×25-cm pan. Mix together the sugar, eggs, and butter in a large bowl using a whisk, hand mixer, or stand mixer. No need to whip them—just mix until combined. 

2. Melt the chocolate in a double boiler or the microwave. Cool slightly. Then add it to the butter mixture and mix until combined. Add the nuts and mix. 

3. Pour into the pan and bake until set, but not dry, about 30 minutes. Cool to room temperature.

Whipped Milk Chocolate Ganache:

4. Heat 300 grams cream in a small pot until simmering. 

5. In a medium bowl, pour the hot cream over the chocolate, wait a few seconds, then stir to make a smooth ganache. Refrigerate for at least 4 hours. 

6. Remove from the fridge, add the remaining cream, and whip until light and airy. Be careful not to over-whip, as the ganache can break. 

7. Pour the ganache over the chocolate cake and smooth using an icing spatula. Refrigerate until firm.

Coffee Mascarpone Cream:

8. Heat the cream until simmering. 

9. Pour the hot cream over the chocolate in a bowl, wait a few seconds, then stir to make a smooth ganache. Refrigerate for about an hour. 

10. Combine the mascarpone and chocolate ganache, add the coffee paste, and mix. Refrigerate for 2 hours. 

11. Remove from the fridge, and whip until stiff peaks form. 

12. Spread the mascarpone cream over the whipped ganache layer, smooth with an icing spatula, and refrigerate until firm.

Dark Chocolate Coffee Ganache: 

13. Combine the cream and coffee and heat until simmering. 

14. Pour the hot cream over the chocolate, wait a few seconds, then stir with a spatula to make a smooth ganache. Stir very carefully without lifting the spatula from the bottom of the bowl in order to avoid incorporating air bubbles. 

15. Pour the ganache over the cake, and shake the pan lightly to spread it or use an icing spatula to smooth and make designs. 

16. Refrigerate for a few hours; then, dig in!