Microwave cream and milk together for 3 minutes, until bubbles appear on the sides. Stir in the chocolate and coffee liquor. Stir until melted. Add egg yolk and mix vigorously. Microwave for 30 seconds. Stir once and microwave again for 30 more seconds. Pour into serving containers while warm. Cover with plastic wrap or foil to prevent the chocolate crème from forming a crust.
Microwave cream until bubbles appear on the sides, about a minute. Stir in the chocolate and the zest. Keep whisking until chocolate completely dissolves and glistens. Cover and refrigerate for 2 hours. When ready, scoop a tsp of the chocolate mixture, roll into a ball in the palm of your hands. Roll either over cocoa powder or chopped nuts. I have used pistachios.
When ready, top one truffle over chocolate crème and serve with dessert spoons. Leave additional truffles on the table and let the guests help themselves.