Method:

1. Fill a shallow pan with 1-1/2 inches water; bring to a boil over medium heat. Place chocolates and butter in a 2-quart metal mixing bowl. Place bowl in the shallow pan of boiling water and turn off heat. Stir to mix chocolates and butter while they melt. 

2. Sift together flour, baking powder, baking soda, and salt onto a piece of aluminum foil or parchment paper. 

3. Beat eggs, sugar, rum, and vanilla into melted chocolate mixture until mixture thickens. Stir in sifted flour mixture and combine thoroughly. Stir in chocolate chips and walnuts. Let mixture sit until easy to shape (5 to 10 minutes); it will be sticky. 

4. Place dough on a sheet of aluminum foil about 18 inches long and roll into a log 2 inches in diameter. (For a more elegant cookie, roll 2 cylinders each about 1 inch in diameter.) Wrap carefully and chill overnight. 

5. Preheat oven to 180 degrees Celsius. Line baking sheets with aluminum foil. Slice cookie dough with a hot knife. Place slices on foil. Bake until cracks form on top of cookies and surfaces appear dry; interiors of cookies as seen through cracks will be moist (10 to 12 minutes). Let cool 5 minutes on baking sheets before removing to a wire rack to finish cooling.