Preheat oven to 350 degrees Fahrenheit.

In a double boiler, microwave, or a pan placed over a pot of boiling water, melt chocolate, mixing until smooth.

Remove from heat and stir in coffee.

In a bowl, beat egg yolks and one-fourth cup of sugar on high speed for about eight minutes, until the mixture becomes a fluffy, light whipped cream.

In a separate bowl, beat egg whites and remaining one-third cup of sugar on medium speed until the mixture becomes a firm foam.

Add melted chocolate to the egg yolk mixture and mix until combined. Add the liqueur.

Add yolk/chocolate mixture to the egg whites, using a spatula to gently combine in a folding motion.

Divide the mixture into two portions: one-third of the mixture and the remaining two-thirds of it. Refrigerate the smaller portion. Add chopped nuts to the larger portion and mix well. Pour into the pan and bake about 45 minutes.

Remove from the oven and refrigerate about 30 minutes.

After the cake has cooled and the center has sunken a little, remove the smaller portion from the refrigerator and gently fill the depression of the cake with it. Freeze at least two hours. (The cake can be stored in the freezer for up to one month.) Move the cake to the refrigerator about 30 minutes before serving.