Dark Chocolate Layer Cake with Chocolate Ganache

This is a sweet and very rich chocolate cake taken from the Moosewood Restaurant Book of Desserts. The authors refer to the frosting as "a day trip to chocolate cream heaven."

Cake
1/2 cups (375ml) boiling water
3/4 cup (175 ml) unsweetened cocoa powder
2 cups (500 ml) unbleached white flour
1 1/4 teaspoons (6 ml) baking soda
1/8 teaspoon (0.5 ml) salt
1 cup butter (250 ml), at room temperature
1 1/2 cups (325 ml) packed brown sugar
2 large eggs
2 teaspoons (10 ml) pure vanilla extract

In a mixing bowl, gradually add a bit of the boiling water to the cocoa, whisking to make a paste, then whisk in the rest of the water. Set aside to cool.
Preheat the oven to 350 degrees F (180 C). Butter two cake pans and lightly dust them with flour.
Sift together the flour, baking soda, and salt and set aside. With an electric mixer, cream the butter and brown sugar. Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract. Add the flour mixture alternately with the cooled cocoa mixture, beating after each addition.
Divide the better evenly between the two prepared pans. Bake for 25 to 30 minutes, until a knife inserted into the centre comes out clean. Cool the layers in the pans for about 5 minutes, then turn them out onto racks to cool completely. Frost with chocolate ganache.

Chocolate Ganache

1 1/2 cups (375 ml) heavy cream
9 ounces (225 g) semi-sweet chocolate
1 tablespoon (15 ml) hazelnut-flavoured liqueur, like Fra Angelico (optional)

In a saucepan, bring the cream to a simmer. Melt the chocolate or chop it into small pieces. Whisk the melted chocolate or chocolate pieces into the simmering cream and continue to heat gently, in necessary, to fully melt the chocolate. Stir in the liqueur, if using.
Pour the chocolate cream into a bowl. Chill for about an hour, stirring every 10 to 15 minutes until it has thickened enough to spread. Store in the refrigerator until needed.