Place the chopped chocolate into a heat proof bowl.
Place the milk, liqueur and sugar into a small saucepan. Heat over medium temperature until the sugar melts and the milk begins to simmer. Stir frequently.
Pour the hot milk over the chocolate. Let sit for 2 minutes, then stir until all the chocolate is melted. Once the chocolate is smooth, pour it onto a baking sheet and spread it into an even layer. Chill in the refrigerator for 20 minutes.
Cut the room temperature butter into cubes and place into an electric mixer. Beat on medium high speed until smooth. This will take about 4-5 minutes. Stop a couple of times to scrape down the bowl. Once the butter is smooth, scrape the cooled chocolate from the baking sheet and put it into the mixer. Beat on high speed until a light fluffy frosting is formed.
Spread the frosting on the cooled cake.
Top with chopped nuts, if you like.
Store the cake in an airtight container in the refrigerator for up to 2 days.