Preheat oven to 350 degrees F (175 degrees C). Grease one 9x13 inch baking pan.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla, peppermint extract, and creme de menthe liqueur. Combine the flour, baking powder, and salt; stir into the creamed mixture alternately with the milk.
Stir in the chocolate chips and walnuts (if using). Spread batter into the prepared pan.
Bake for 30 to 35 minutes in the preheated oven. Cake is done when it springs back to the touch. Cool before frosting.
Interesting Facts
Creme de menthe is a sweet, mint-flavored liqueur made from a combination of peppermint, spearmint, and other herbs.
This cake is a great dessert to make for St. Patrick's Day.