Add the chopped chocolate, butter and coffee to a small sauce pan and set over low heat. Gently melt the chocolate and butter together until melted and smooth, stirring often. Be sure to keep the heat on the lowest setting possible to ensure you do not burn the chocolate. You may also melt the chocolate in the microwave, stirring every 30 seconds until melted and smooth. Once the chocolate is melted, remove from the heat and set aside to cool.
In a small sauce pan whisk together the four egg yolks, sugar, whiskey and 1 tablespoon milk. Place on the stove and set over medium-low heat, whisking constantly until the sauce thickens slightly and just coats the back of a wooden spoon, about 2-3 minutes. Stir the melted chocolate into the egg mixture until completely smooth. If there are any clumps of egg, strain the mixture through a fine mesh strainer. Add the remaining 4 tablespoons milk and place in the fridge while you whip the eggs whites.
Using a mixer, whisk the egg whites with a pinch of salt until just stiff and frothy. Add 1 tablespoon of sugar and beat again until just glossy.
Using a large spatula, add one spoonful of egg whites to the cooled chocolate mixture and fold it in gently. Gradually fold in the remaining egg whites, being careful not to over mix the mixture.
Divide the mousse between individual serving jars or glasses. Place in the fridge and chill for 3–4 hours before serving.
Just before serving, use and electric mixer to whip the cream with the bailey's until soft peaks form. Serve the mousse topped with a thick layer of whipped cream.
*Adapted from What Katie Ate by [Katie Quinn Davis | http://www.whatkatieate.com/].