Using a hand mixer, cream the egg yolks and 1/3 cup of sugar until pale and fluffy. Reserve.

In a saucepan, over medium heat, combine the milk and the remaining 1/3 cup of sugar. Once it begins to simmer and the sugar is dissolved, remove from heat and pour a small amount in the creamed yolks, whisking constantly so you don't end up with sweet scrambled eggs. Once the creamed yolks are tempered, add it to the saucepan and bring it back to the stove.

Continue cooking, stirring constantly, until it thickens slightly and the custard coats the back of a spoon and a line drawn with your finger comes clean. Reserve.

Melt the chocolate by placing it in a medium sized bowl and putting it in the microwave. Heat it on medium high for about 1 minute, then remove from the microwave and stir. Repeat heating at shorter intervals of 15 seconds, stirring in between, until the chocolate is completely melted and smooth. Alternatively, you can melt it in a double boiler on your stove.

Combine the melted chocolate, the custard and the liquor, if using. Reserve.

Using your hand (or stand) mixer, whip the heavy cream until soft peaks form.

Add 1/3 of the whipped cream to the chocolate/custard batter and whisk by hand, vigorously, until combined. Then, gently fold in the remaining whipped cream, making sure not to deflate the mousse.

Transfer the mousse to a large serving dish or individual glasses and refrigerate for at least one hour to set.

Serve cold. (I like to let it stand at room temperature for 5 minutes before serving!)