Sift flour and salt into a mixing bowl. Melt chocolate in a bowl placed over a saucepan of hot water.
In a separate bowl, cream butter and sugar together until fluffy, then beat in the chocolate and mashed potato.
Gradually beat in the egg, adding a little flour with each addition.
Fold in the rest of the flour and stir in the milk.
Divide mixture between cake tins and bake for 25 to 30 minutes or until top is firm but springy to the touch.
Remove from oven and after a few minutes, turn out on a cooling rack.
While the cake is cooling, make the icing. Melt the chocolate as before, stir in the other ingredients and mix well.
Use the icing to sandwich the cake layers together and coat the top and sides of the cake.
HINT: What makes this rich chocolate cake "Irish" is not only to the creamy liqueur used in the filling, but also a particularly "Irish" ingredient in the cake mixture itself, which makes it extremely moist. Have you guessed it yet? Yes, it's the potato! Mystery solved! Bet you were wondering what potatoes were doing in a cake recipe! Even Irish dessert recipes make use of this abundant Irish crop!